emt project 2

Introduction to the Electromagnetic Spectrum

Editor:

Daniel Finkenthal

Written by:

Daniel Finkenthal

Beverly Greco

Rick Halsey

Lori Pena

Steve Rodecker

Billy Simms

Rick L. Lee

John Lohr

Mike J. Schaffer

David P. Schissel

QTYUIOP

The Electromagnetic SpectrumContents

iii

Table of Contents

Introduction Instructor/Student 1 The Visible Electromagnetic Spectrum ........................................................ 1 ....... 7 2 Invisible Regions of the Electromagnetic Spectrum .................................... 9 .......13

Visible Light 3a Why Are There Colors in a Compact Disk? .................................................15 .......17 3b The Compact Disk as Diffraction Grating ....................................................15 .......19 4 Measuring Wavelengths of Light .................................................................21 .......23 5 Young’s Experiment .....................................................................................25 .......27

Sunlight 6 Blue Skies and Red Sunsets .........................................................................29 .......31 7 Photosynthesis...............................................................................................33 .......37

Infra-Red Radiation 8 Infrared Radiation and the Inverse-Square Rule...........................................39 .......41 9 Detecting Infrared Radiation Using a Prism ................................................39 .......4310a Investigation of IR Light Using an IR Transmitter and Receiver ................45 .......4710b Investigation of IR Light Using a Close Circuit TV Camera ......................45 .......49

Ultra-Violet Radiation11 Fluorescence .................................................................................................51 .......5312 UV Light Detection ......................................................................................55 .......5713 Investigating the Absorption of UV Light by Oxygen .................................55 .......5916 The Effect of UV Light on Yeast .................................................................61 .......6315 The Effect of UV Light on DNA ..................................................................65 .......6716 Which Wavelength Causes Photogray Lenses to Change Color?.................69 .......7117 Which Wavelength Causes Sunrez® to Solidify? ........................................73........75

Radio and Micro-Waves18 Measuring the Length of Radio Waves ........................................................77 .......7919 Tuning Into Radio Waves .............................................................................77 .......8120 The Shielding of Radio Waves by Metal......................................................77 .......8321 Using the Earth’s Ionosphere to Reflect Radio Waves.................................77 .......8522 A Diffraction Grating for Radio Waves .......................................................77 .......8723 A Diffraction Grating for Microwaves .........................................................77 .......89

Contents

The Electromagnetic SpectrumOverview

continued

Curriculum Overview:

Introduction to theElectromagnetic Spectrum

In the matter of physics, the first lessons should contain nothing but what isexperimental and interesting to see. A pretty experiment is in itself often morevaluable than 20 formulae extracted from our minds; it is particularly important thata young mind that has yet to find its way about in the world of phenomena should bespared from formulae altogether.

– Albert Einstein

OverviewThis section focuses on activities that help students understand the electromagnetic spectrum, one

of the six stations on the DIII-D Tokamak Fusion Facility tour at General Atomics in San Diego,California.. The goal is to help teachers teach the often difficult concepts related to the electro-magnetic spectrum as well as prepare students for the tour. This section contains a number of more orless informal laboratory units including demonstrations, experiments, and activities that are unlikelyto be found in traditional science textbooks or lab manuals. Each unit contains an Instructor’s Guideand a master copy of a Student Activity Sheet to be reproduced and distributed to each studentparticipating in the unit.

Mission StatementThe curriculum contained here was developed by local teachers and scientists working together to

improve the state of science education in today’s schools. The aim is to increase the understandingand enthusiasm for science in high-schools through the use of more enlightening, empowering, andsocially relevant curriculum. We hope to help students understand and master the technological worldaround them in order to increase their own sense of power and control over their lives. By increasingunderstanding we seek to reduce the mystification, powerlessness and alienation of people fromscience, and eliminate the sense of elitism associated with science. These are lofty goals, and we hopeto rise to the challenge.

ContentsThe curriculum units have been grouped into six different sections depending on their respective

emphasis. These sections are named as follows:

• Introduction • Infrared Radiation• Visible Light • Ultraviolet Light• Sunlight • Radio and Microwaves

A complete listing of the units in each section is given in the Table of Contents that follows.

FormatFrom collective meetings and discussions with teachers at various levels, an optimized format for

presenting each curriculum unit was devised. Each unit includes a master copy of a single double-sided Student Activity Sheet, organized according to the table below. It was decided early on torestrain each Activity Sheet to a single double sided page since many teachers feel that anymoreoverwhelms the student or tends to get lost in the hustle, bustle, and shuffle of a typical school day.

Each Student Activity Sheet is accompanied with an Instructor’s Guide. The Instructor’s Guidecontains stated goals and objectives along with background information, helpful hints and availableresources, and ideas for further investigation. In most cases a complete description of each unit iscontained in the Student Activity Sheet, while the Instructors Guide is intended to serve as an aid forthe instructor organizing the activity at hand. In the case of laboratory demonstrations, however, thebulk of the material is contained in the Instructor’s Guide.

A master copy for reproduction of each Student Activity Sheet directly follows each Instructor’sGuide unit. A second set of Student Activity Sheets are also grouped together in a separatelyorganized Student Activity Handbook. Teachers may wish to have the handbook duplicated as awhole.

Overview

Curriculum Overview: Introduction to the Electromagnetic Spectrum

Table 1 Organizational format of each curriculum unit

Instructor’s Guide: Student Activity Sheet:

Goals Purpose

Objectives Required Equipment

Background Information Discussion

Helpful Hints Review Questions

Extensions Procedure

References Analysis Questions

Resource Box

A Resource Box containing the more unusual, expensive, or hard-to-obtain items involved in eachof the Activity Units has been developed and assembled by the DIII–D Tokamak Fusion group to bedistributed with this Curriculum. The more common classroom items such as an overhead projector,paper, tape, etc. are assumed to be available and will not be included in the Resource Box. Since theavailability of many materials varies with each school, please evaluate the Required Equipment andSupplies list in each activity and note what is and is not available at your school.

Four Resource Boxes have been assembled and placed at different San Diego county schools tofacilitate distribution to local teachers. Each of these participating schools is charges with loaning andmaintaining an individual Resource Box. A Resource Box may be obtained by contacting one of thefollowing teachers:

Rick Halsey Scripps Ranch High School (621-9020)

Lori Pena Roosevelt Junior High School (293-8675)

Steve Rodecker Chula Vista High School (691-5439)

Billy Simms La Jolla Country Day School (453-3440 x169)

This curriculum is an evolving work and needs your input. Evaluations and comments can besubmitted to the Fusion Education Curriculum Web page or to Dr. Daniel Finkenthal at (619) 455-4135, E-mail to [email protected]. Periodic updates will also be made available at the Web site:

http://FusionEd.gat.com/

© General Atomics 1996

The Electromagnetic SpectrumIntroduction–Visible Light

1

Instructor’s Guide to Lab No. 1:

The Visible Electromagnetic SpectrumGoalThe goal is to introduce the visible electromagnetic spectrum to students through use ofmaterials readily available to most high school science classes.

ObjectivesAfter observing these demonstrations, students should be able to:

• Use a diffraction grating to separate a visible light source into its component parts.• Explain what a continuous emission spectrum is and give several examples.• Explain what a bright line emission spectrum is and give several examples.• Explain what an absorption spectrum is and give several examples.• Relate the color of viewed objects to both the wavelengths of light incident upon it and

the wavelength of light it absorbs/reflects.

Background Information

The phrase “electromagnetic spectrum” is frequently referred to in the study of science.In biology it is often a part of the discussion of photosynthesis, the physiology of the eye, andmutagenic sources. In earth and space science electromagnetic radiation is often a part of adiscussion of radioactive minerals, cosmic rays being deflected by the earth's magnetic field,and analyzing incoming radiation from stars by optical and radio telescopes or other means.In chemistry the spectrum is often discussed when talking about evidence for differentelectron energy levels and characteristic properties of elements. In physics it is a part of thestudy of waves, electricity and magnetism, and modern physics. The table on the followingpage divides the electromagnetic spectrum into eight bands by common names although thedifferences between types are gradual rather than discrete.

Helpful Hints1. Timeline

One to two class periods depending on which spectrasources are chosen.

2. Overhead Projector Setup

A. A mask, made from two pieces of cardboard, forexample, can be used to block out all but a narrow(2 to 4 cm) slit of light coming from the projector. Tape the large holographicdiffraction grating (see description below) to a frame of cardboard and tape thisframe to the focusing lens of the overhead. The grating should be mounted at an angleof about 20° from the head. Darkening the room will increase contrast; decreasing theslit width dims the intensity but increases the dispersion (color separation). Thebrighter the overhead, the brighter the spectrum will be. Overheads used to projectLCD panels, typically 4000 lumens or more, are especially good. Use of a moviescreen rather than a white board (marker/dry erase board) is preferable.

Demonstration

OverheadProjector

diffractiongrating(∠ 20°)

maskwith slit

2 Instructor’s Guide to Lab No. 1

2

B. A holographic diffraction grating is a transmission phase delay grating whichproduces a very bright first order image (energy transmitted to the zero order isminimized by maximizing interference at the center of visible spectrum at about550 nm). The cost4 is about $6 each. Forty of these are included in each RB, one perstudent. Most inexpensive student hand-held spectroscopes are less dispersive andproduce a less intense first order image.

C. A grating produces a number of repeating spectra on either side of the central image(also called the zero order, which is an image of the source), and they are called firstorder, second order, etc. The zero order and one or more orders can be shown at thesame time, or by adjusting the angle that the overhead projector has with respect tothe screen, and by adjusting the projector to screen distance, one can show just onebright first order spectrum. If a meter stick is taped to the screen and the correctprojector to screen distance is chosen, the 40 to 70 cm range on the meter stick canrepresent the 400 to 700 nm visible spectrum for rough estimates of wavelength. Or, along sheet of paper may be taped to the screen, and the appropriate intervals markedoff. The brightness of the projected spectrum can be maximized by rotating (in avertical plane) one edge of the grating away from the lens to about 20°.

NAMEλ RANGE

(m)f RANGE

(Hz)ORIGIN/CAUSE

INTERESTINGFACTS

USES / RELATEDCAREERS

ElectricPower

>105 <102

vibrating atomsor molecules

overmacroscopic

distances

60 Hz hum heardnear electric trans-

formers

transmits electric energy tohomes from power

stations; electrician;electrical engineer

Radio/TV 10-1-104 109 - 104

vibrating atomsor molecules

overmacroscopic

distances

low frequenciesare reflected by

earth's atmosphere

Radio and TV; electricalengineer; communications

industry, medicine,magnetic resonance

imagingmicrowave

10-3 - 10-1 1011 - 109vibrating atomsor molecules

these waves areblocked by "dots"on µλ oven doors

cooking; long distance TVand phone; radar; terrain

mapping

infrared(IR)

10-7 - 10-3 1014 -109vibrating atoms

or electrontransitions

passes throughhaze in theatmosphere

heating & drying; "nightvision" cameras; TV &garage door remotes;

satellite remote sensing

visible 4-7 x 10-77.5x1014-

4.3x1014vibrating atoms


about 1/40 of totalEMR spectrum

what the eye and typicalfilm can “see”; optometrist

ultraviolet(UV)

10-8- 7x10-7 1016 -1014vibrating atoms


"burning" rays ofsun;

germicidal, photo-chemical, photo-electric

effects; hardening casts inmedicine

X-ray 10-11- 10-8 1019 - 1016electron

transitions andbraking

λ is size of atommedicine; crystallography;

astrophysicist; remotesensing

gamma <10-11 >1019 nucleartransition

can cause tissuedamage andionization

research into structure ofnucleus; geophysics;mineral exploration

The Visible Electromagnetic Spectrum 3

3

3. Spectra SourcesA. Emission Spectrum - Continuous

Use the overhead projector as described above.

• Complete spectrum - You should be able to obtain a single bright spectrum ifyou have aimed the projector off-center and rotated the grating at about 20o. Thewidth of the opening on the overhead projector platen has some effect onintensity.

B. Emission Spectrum - Bright LineRemove the large grating from theoverhead. An entire class can view thespectrum from the following twosources if light from the source ispassed through the large grating.

• Flame Test – Spectra are produced by heating salt solutions in a clean burningBunsen flame. A wire loop is cleaned by inserting it into hydrochloric acid andthen heated to incandescence in the flame until no color shows in the flame. Thenit is placed into the test salt and held in the burner flame. The salts typically burnquickly so students must be alert to pick out the lines. You can expect thefollowing lines to be observed: Calcium chloride (orange-yellow), potassiumchloride (violet), sodium chloride (yellow), and strontium chloride (red).

• Gas Discharge Tube – A hot gas under low pressure will emit certain bright linesthat can be distinguished with the grating. The gas tubes are placed across highvoltage which ionizes the gas. Commonly available gases include hydrogen,helium and neon. Neon is particularly good because of its brightness. This is avery good way to illustrate the differences in electron energies. However neitherthe spectrum tubes ($20 and up) nor the power supplies ($125 and up) arecommon outside the physics classroom. You can expect hydrogen to produce atleast one violet, one blue-green, and one red band that can be seen. Neon has overa dozen bands, mostly in the yellow to red end of the spectrum. Helium has bandsin the violet, blue, blue green, yellow, and red.

• Mercury Discharge Lamp – A hot mercury gas under low pressure emitsdistinct bright lines of several wavelenghts that can be distinguished with thegrating. You can expect to see violet, blue, blue- green, green, yellow, and orange.

The following sources are generally tooweak to be projected for an entire class, butare visible if each student has their ownsmall grating5 and views the source in adarkened room. Each emits one or morebands of light. One possible method ofdemonstrating these spectra is to havestations set up around the room and to havestudents visit each station and record theirobservations.

• Cyalume Sticks (available in several colors in diving shops and some toy storesfor $1-$2 each) Expected Outcomes: The green emits strongly in the green butalso has some yellow and blue; the red produces most strongly in the red, orange,and yellow, with some green.

• Neon bulbs ($10-$20, available in specialty lighting stores) and GrowLux©

fluorescent lamps ($2-$4, available in gardening supply stores) – Neon has over a

source

light

studentslargegrating

source & slit

light

studentssmallgrating


4

dozen bands, mostly in the yellow to red end of the spectrum. GrowLux© hasmany bands, especially red, and blue/violet.

• LEDs of various colors (available from electronics supply shops such as RadioShack for $1-$2; they are current limited so must be wired in series with a resistorwhen attached to a battery or power supply) - red, yellow, and green emitters areavailable. Expected Outcome: some narrow band red, yellow, and green emittersare available that emit only those colors but common LEDs usually emit stronglyin one region and weakly in others.

• Blacklight fluorescent lamps - violet, green, and yellow lines can be expected.

• Fluorescent Crayons - Illuminate with the UV sources. Each color will producea variety of lines.

C. Absorption Spectra

Set up the overhead projector with the grating mask as in the first demonstration.Maximize separation. Use mirrors to mix colors; show the effect of absorption byblocking one or more colors.1. Hold a chlorophyll solution in portions of the projected spectrum and note that

bands of the spectrum are absorbed. This solution can be made by placing ahandful or two of fresh green plant matter, such as grass clippings, in a blender.Add alcohol, blend, and filter. You will have to play around with the concen–tration to obtain the desired effect. Several wavelengths in both the red and blueend of the spectrum are absorbed so the resulting spectrum will appear dimmed inthese regions compared to white light.

2. Hold glassblower glasses (neodymium and praseodymium, sometimes calleddidymium) in portions of the projected spectrum and note that a band in theyellow is strongly absorbed; bands in the violet, blue, green, orange, and red areabsorbed to a lesser extent. Neodymium salts will also produce this effect.

3. Cobalt glass , used for welders glasses and in various physics experimentsstrongly absorbs yellow and orange, plus some red and green).

4. Hold a beaker of Vanish© Toilet Bowl Cleaner in portions of the projectedspectrum and note that it strongly absorbs in the mid red range. The crystalproduct is not the same as the liquid, so be sure to buy liquid Vanish. If thesolution is held in a white light source, the projected light appears blue and onlyslightly dimmed because the red absorption band is narrow. However, if ahelium-neon laser is pointed at the solution, the absorption is almost complete asthe emitted wavelength of a He-Ne laser is near the middle of the range offrequencies absorbed by Vanish.

5. Hold colored plastic sheets, especially theatrical lighting gels , in portions of theprojected spectrum and note absorption bands. Or place the gels directly on theplaten. Depending on the quality of the gel, absorption can be narrow or broad. Agood quality red filter will pass only red and orange and possibly a faint bit ofyellow. Poor filters may strongly pass the red end but weakly transmit the rest.

6. Take a fluorescent (glow-in-the-dark) marking pen and remove the cap; soak thetip in about 50 ml of alcohol overnight. This solution absorbs in the green and red.Dilute solutions of water-soluble fluorescent paints also work. In either case, besure to read the label carefully before buying as some pens and paints are calledfluorescent when in fact they are bright colors. True fluorescent materials must besubjected to either long or short wave UV (from the sun or UV lamps) before theywork. Note: The “Invisible Ink” included in the activity kit can be used here.


5

D. Combined Spectra An especially effective display can be madeby comparing the spectra formed by severalof the above demonstrations at the sametime. For example, place a red gel, a greengel, the cobalt glass, and the fluorescent dyeone below the other on the platen of theoverhead with a narrow strip of cardboardbetween each (to set off the spectra whenprojected). If space for white light is madeavailable at the top and bottom of thissequence, a comparison can be made to afull spectrum.

4. Color TheoryNote: The following two activities require a bright projector and a dark room to beeffective.

A. Reflection Characteristics Have students view different colored objects, such as sheets of colored paper, inwhite light; then have them observe the same objects in portions of the spectrumproduced by the overhead projector and grating. Note the color that the object appearsto be depends on both the color of the incident light as well as the “color” of objectitself. Have students predict the color of test objects.

B. Color Mixing Mirrors placed in the path of the projected spectrum and aimed so that light ofdifferent colors falls on the same spot allow students to experiment with the effects ofcolor mixing. Students are sometimes surprised that certain colors such as yellow canbe produced without any yellow at all by mixing green and red!

5. Polarization of Light

A. Use the OHP to show the polarization of light.| Place a single sheet of polarizing film (Polaroid ) on the projector and have studentsnote the decreased transmission. Place a second sheet on top of the first and rotate toshow transmission and absorbtion. Next place a third sheet diagonally in between twocrossed polarizers. Several polarizing filters are included in the resource box.

B. Colors from Cellophane Place some crumpled cellophane over one of the polaroids on the OHP (also trystrips of cellophane tape overlapped at different angles, and experiment with differentbrands of transparent tape). Project the image onto a screen and rotate a second,slightly large Polaroid in front of the OHP lens. Do this in rhythm with some “hip”music and you’ll have a spectacular light show.

C. Colors from Karo Syrup Place a bottle of Karo (corn syrup) between two sheets of polaroid, and place a whitelight source behind the seemingly-clear syrup. Have students view through thePolaroids and syrup and view the spectacular colors as you rotate one of thepolarizers. This can also be projected onto a screen using OHP with a flat container ofKaro, such as a petri dish.

D. Calcite crystal

A calcite crystal can be used as above to demonstrate another more complicatedpolarization effect resulting from a combination of double refraction and bifringence.

red

open

blueholmiumchloride

opencardboard mask

platen (slit)


6

E. Liquid Crystal Displays The common LCDs used all over the place in calculators, watches, clocks, etc., makeuse of the polarization of light using a liquid crystal that changes polarization with anapplied voltage. Have students examine a typical display through a Polaroid. Havethem rotate the Polaroid to ensure they get the full effect.

F. Polarized Light MicroscopeStudents can view spectacular interference colors with this setup. Any microscope,including an inexpensive toy microscope, can be converted into a polarized-lightmicroscope by fitting a piece of Polaroid inside the eyepiece and taping another ontothe stage of the microscope. Mix drops of naphthalene and benzene on a slide andwatch the growth of crystals. Rotate the eyepiece and change the colors. Or simplyobserve the interference colors of pieces of cellophane or transparent tape.

6. Blue Skies and Red SunsetsThis activity is presented in Laboratory 6. It is worth doing as a quick demo using ifyou do not have the time for the formal lab activity. Use a slide projector for bestresults. It also makes a good home project, since it only needs a flashlight and a largeglass bowl.

7. Interference by Thin FilmsYou can do this one as a demo or assign it as a home project in the kitchen sink. Dip adark-colored coffee cup (dark colors make the best background for viewing inter-ference colors) in dish washing detergent, and then hold it sideways and look at thereflected light from the soap film that covers its mouth. Swirling colors appear as thesoap runs down to form a wedge that grows thicker at the bottom with time. The topbecomes thinner, so thin that it appears black. This tell us that its thickness is lessthan one-fourth the thickness of the shortest waves of visible light. Whatever itswavelength, light reflecting from the outer surface reverses phase, rejoins lightreflecting from the inner surface which doesn’t reverses phase and cancels. The filmsoon becomes so thin it pops.

References This apple means that a particular item is included in the Resource Box.

1. Philip Sadler, "Projecting Spectra for Classroom Investigations," Physics Teacher 29,423 (1991).

2. George A. Burman, "Overhead Spectroscopy," Physics Teacher 29, 470 (1991).

3. Kenneth Brecher, "Do Atoms Really 'Emit' Absorption Lines? ," Physics Teacher 29, 454(1991).

4. Learning Technologies Inc., 59 Walden St., Cambridge, MA 02140 (617) 547-7724. One4.5" x 5" sheet with four color filters costs $6 plus shipping and handling. Included inResource Box .

5. Holographic diffraction gratings enclosed in 35 mm glass slide mounts are available for$3 each from Arbor Scientific, P.O. Box 2750, Ann Arbor, MI 48106-2750, (800) 367-6695. These have been included in the Resource Box, enough for an entire classroom.

The Electromagnetic SpectrumIntroduction–Visible Light

7

Laboratory No.1:

The Visible Electromagnetic SpectrumPurposeThe purpose is to investigate the visible electromagnetic spectrum using a diffraction gratingto observe different light sources.

Required Equipment and SuppliesDiffraction gratings or hand-held spectroscopes; overhead projector with mounted largeholographic diffraction grating; various light sources.

Discussion

Energy emitted from vibrating electric charges produces electromagnetic waves. Our eyes aresensitive to just a small portion of the electromagnetic spectrum. The sun, normal incan-descent bulbs, and most fluorescent bulbs produce nearly white light by mixing all thefrequencies (colors) together. White light can be separated into its component colors, called aspectrum, by passing the light through a prism or a diffraction grating. If a light sourceproduces all the visible frequencies (such as the sun), the spectrum is called a continuousemission spectrum. If the source produces only certain frequencies (such as a gas at lowpressure, a neon sign for example), the resulting spectrum is called a bright line emissionspectrum. If a transparent substance (such as stained glass) absorbs or removes certainfrequencies from white light, the spectrum produced is called an absorption spectrum.

Review Questions1. What causes electromagnetic waves?2. What event causes visible light to be produced?3. What are the three types of spectra?4. Do atoms emit absorption lines?5. Does the color of a banana change if the color of light hitting it changes?

Procedure

The instructor will set up a number of different sources of light, producing a variety ofspectra. Observe each of these sources through a diffraction grating, and record the sourcename and observations for each of the spectra types.

Describe your findings for the following demonstrations:

1. Continuous emission spectrum –This is produced using the “white hot”incandenscent light bulb inside an overheadprojector, which is masked to allow only anarrow slit of light which is then passedthrough a a large diffraction grating afterbeing focused by the projector lens.

Demonstration

Name: Class: Date:

OverheadProjector

diffractiongrating

maskwith slit

Overhead Projector Setup

8 The Visible Electromagnetic Spectrum Laboratory No. 1

8

2. Bright line emission spectrum –A number of different emissison spectrawill be produced, including:a. flame testsb. gas discharge tubesc. Cyalume sticksd. LEDse. blacklightf. neon bulbg. GrowLuxh. fluorescent Crayons

3. Absorption spectrum –Several different absorption spectra are to be demonstrated, including:a. chlorophyllb. didymium glassc. colbalt glassd. holmium chloridee. Vanishf. lighting gelsg. fluorescent dye

Analysis of Experiment1. What physical event produces the different frequencies that make up a continuous

spectrum? What evidence do you have from this investigation that your answer iscorrect?

2. If white light is passed through a cloud of sodium gas and then dispersed by a grating,certain frequencies at about 590 nm are removed. What frequencies do you think wouldbe emitted by sodium if it were heated to high temperature? Why?

3. Is the color of an object always the same? What evidence do you have from thisinvestigation? What color would a red dress appear if viewed under blue light?

4. Do filters (gels) turn one color of light into another color of light? For example wouldgreen light turn into blue if passed through a blue filter? What evidence do you havefrom this investigation?

5. Can you produce yellow light without using any yellow? Explain.6. Could spectral analysis (such as the flame test or placing the chlorophyll in the light path)

help you identify an unknown object? Explain. Could this be used in forensics?

source

light

studentslargegrating

Observation of Light Source through Grating(Lab Station)

The Electromagnetic SpectrumIntroduction–Invisible Regions

9


Demonstrating the Invisible Regions of theElectromagnetic Spectrum

GoalThe goal is to outline techniques for demonstrating the invisible regions of the electromagneticspectrum using materials commonly available to high school physics courses.

ObjectivesAfter observing these demonstrations, students should be able to:• Explain that visible light is only a small portion of the electromagnetic spectrum;• Recognize that the spectrum produced by a blackbody radiator is related to the temperature of the

radiator;• Use an ultraviolet viewer or UV sensitive liquid crystal to detect the presence of UV rays in

common electromagnetic radiation sources;• Recognize that infrared waves transfer energy by waves which have properties similar to light

waves such as reflection, refraction, and diffraction;• Explain that radio and TV signals are waves which can be shielded;• Demonstrate that microwaves are larger than the interhole distance on a microwave oven door

and explain that microwave heating is due to two radiation effects;


See background information on visible electromagnetic radiation. The phrase “electro-magnetic spectrum” is frequently referred to in the study of science. In biology it is often apart of the discussion of photosynthesis, the physiology of the eye, and mutatgenic sources.In earth and space science electromagnetic radiation is often a part of a discussion ofradioactive minerals, cosmic rays being deflected by the earth's magnetic field: analyzingincoming radiation from stars by optical and radio telescopes or other means. In chemistrythe spectrum is often discussed when talking about evidence for different electron energylevels and characteristic properties of elements. In physics it is a part of the study of waves,electricity and magnetism, and modern physics. The table on the following page divides theelectromagnetic spectrum into eight bands by common names although the differencesbetween types are gradual rather than precipitous.

Helpful Hints1. Blackbody Radiation:

The variation of the spectrum produced by a heated object, called blackbodyradiation, can be demonstrated with a variac (a variable ac power supply), a 100-200 Wclear glass bulb ($3–$4), and a porcelain socket ($2). As an alternative to the variac, anin-circuit dimmer extension cord ($15–$20), or a light switch dimmer ($5–$6 if you cando some wiring) can be used. The dimmer can be obtained from a hardware or specialtylighting store. Produce a narrow band of light from your source to reduce stray reflectionsby placing it inside a large can, such as a 48 oz juice can, with a narrow slit cut inone side. Allow for ventilation. Be careful as the can will get hot! Pass the light througha large diffraction grating1 (as discussed in the activity Demonstrating the VisibleElectromagnetic Spectrum) and project the resulting spectrum upon a screen. Vary the

Demonstration


10

temperature of the filament by varying the voltage; note the change in the spectrum. Turnup the voltage until the filament just starts to glow red, and then back it off to the pointwhere the filament no longer glows red. Have students view the spectrum as the voltageis changed. They should note that as the voltage (and therefore temperature) increases, sodoes the intensity of the blue end of the spectrum, indicating more energy being radiated.

Note: A triac-type ac dimmer is included in the Resource Box for this demonstration. It is wiredinto a three-prong socket and cord. It is to be used with the High-Power Light Source (FreyScientific) also included in the RB, which consists of a socket, clamp, chord, and cardboardshroud. Fashion a slit out of cardboard to place in front of the hole in the shroud, held togetherwith elastic bands. Even better is to mount the slit in the end of a cardboard tube, the other ofwhich is mounted to the shroud of the light source.

2. Ultra-Violet Radiation:Both infrared and ultraviolet light can be detected by UV or IR sensitive digital

cameras, but they cost tens of thousands of dollars. UV, IR, and x–rays can be detectedby films obtained from photography supply stores, but there is the cost and delay ofdevelopment. Ultraviolet can however, be detected with a simple filter2 costing about$30–$40. A credit card-sized UV detector3 is also available for about $5 which reacts inseconds to UV and indicates a relative intensity level on a liquid crystal strip. Studentscan use either of these to detect variations in UV production from the blackbody radiator(Demonstration No. 1) and from many other sources such as the sun, TVs, computermonitors, burner flames, and stoves. Show off the fluorescent mineral set, crayons, andblack light poster included in the resource box using the long-wave UV lamp alsoincluded.

Note: Both the UV bandpass filter and several card-size detectors have been included in theResource Box. The filter is to be assembled with the enclosed PVC tubes to form the UV detector.Details are in the Resource Box.

3. Infrared RadiationThe best method we have found for demonstrating IR is using an ordinary CCD or

viticon-based TV camera. The semiconductor detector chips in some of these are verysensitive to IR. We have obtained a number of surplus security cameras, and included onein each Resource Box. Point any type of IR-based remote control unit at the camera and itappears as a bright source on the monitor. Also use the included “IR Flashlight” (an infra-red LED wired into an ordinary flashlight in place of the normal bulb) with the includedprism and/or diffraction grating to show diffraction. Use developed color film as an IRbandpass filter to filter out excess visible light. The lights can be turned out and the IRflashlight can be used to illuminate students. Watch out, you can be observed in thedarkest of nights! A lot can be done with this set up. Be imaginative, and let us knowwhat you come up with!

The TV camera can be used with the optical bench (meterstick type) and diffractiongrating included in the resource box to demonstrate Laboratory No. 4 on the TV monitor.First use the mercury lamp to demonstrate and measure the various visible lines, thenreplace the mercury source with the infra-red LED light source that has been put togetherfor this purpose. The “invisible” lines can be clearly seen on the monitor. Again, thedeveloped color film can be used to screen out most of the unwanted visible wavelengths.

The wave properties of IR can be demonstrated using toy ray guns.4 The Photon© raygun, made by Entertech, or the Bravestarr Evil Laser-Fire Backpack© by Mattel can be


11

used. A homemade version5 can be made for about $5 using a TV remote control as thesignal, and an infrared photo transistor, resistor, LED, and battery as the detector. Thedetails of the detector are given in the appendix. The simplest demonstration involvesaiming the IR beam into a mirror and having it reflect onto the target sensor. In the caseof the Photon gun, the target has a light which changes from green to red when hit withIR. Other demonstrations illustrating wave properties such as the focusing effect ofparabolic mirrors, refraction through plastic prisms, total internal reflection in thickLucite bars, and diffraction around sharp objects can be demonstrated. Note: The “LightListener” described below can be used for these demos.

A “Light Listener” that responds to IR can be built, and plans are included in TheInstructor’s Guide to Lab No. 6 (p. 46). It is the receiving end of a simple amplitude-modulated light wave communications system. It can be used to “listen” to the signalsproduced by light sources such as an IR remote control. Students can use it to hear whattheir TV remote control is saying (a series of tones.) An incandescent lamp will produce ahum, a flourescent lamp a buzz, and an electronic camera flash will produce a large pop.A flashlight beam can be swept slowly across the light listener’s detector to produce asoft swishing sound, while a fast sweep will produce pops. Tap the flashlight with apencil and a ringing sound will be heard as the filament vibrates. Interesting!

Yet another approach is to use an infrared heat lamp, an IR filter,6 and a radiometer.The heat lamp will cause the radiometer to rotate; placing the IR filter between the lampand the radiometer significantly reduces the IR flow and causes the radiometer to slow orstop. Some slide projectors have these glass IR filters between the lamp and the slide toreduce heat transfer to the slide. IR bandpass filters6 which allow only a narrow band ofIR to pass through the filter are also available. This filter can be placed in front of lightfrom an incandescent lamp or the sun, blocking the visible light and allowing only the IR.This filter can be used, for example, by placing the radiometer in a box with a hole cut init the size of the filter. A heat lamp or other bright incandescent source can be placed infront of the filter causing the radiometer to rotate. Viewed from overhead, the boxremains relatively dark, yet the radiometer vanes rotate. Again, depending on the setup,developed color film may work here.

4. Radio and TV WavesMany of these demonstrations are included as individual student activities in the “Radio

Waves and Microwaves (Labs 18–23).” You may wish to demo some here and save some for thestudents to do themselves.

The wave properties commonly associated with light can also be illustrated with radioand TV signals. AM waves are reflected by the ionosphere and therefore can travel 1000sof kilometers. Demonstrate this by tuning in a station from a distant city on an AM radio(this can be a somewhat unreliable demonstration as the heights of the various ion layersvary with weather, time of day, and particle production by the sun). The shielding ofradio waves can be demonstrated by placing a playing radio or TV inside a wire meshcage made from window screen or a metal box. Note that the signal dies away.

5. RadiationSome wave properties can be even more easily illustrated with microwaves than with

light waves but microwave generators and detectors are expensive. However, theubiquitous microwave oven can be used to illustrate some wave properties. An oftenasked question is, “Why can we see through a microwave oven door but the microwaves


12

don't come out?” The explanation is the same one that explains why a sieve allows thesand in a sand and rock mix to pass through but not the rocks. The rocks are too big.Microwaves have a wavelength of about 12 cm, much larger than the inter hole distancein the screen of the door that is but a few mm. The oven can be used to investigatevarious calorimetric variables7 such as efficiency of the magnetron power tube, specificheat of different liquids, oven parameters etc.

The heating effect of microwave ovens is due primarily to its ability to cause watermolecules to vibrate (dipole rotation). There is a secondary absorption method calledionic conduction8. This effect can be demonstrated by comparing the times to boiling ofequal masses of pure water and salted water. The salted water will heat much faster in themicrowave even though its boiling point is higher due to the increased energy absorptionby the ions. Heated on the stove, the salted water takes more time than the pure water toreach boiling!

References:1. Learning Technologies Inc., 59 Walden St., Cambridge, MA 02140, (617) 547-7724. One

4.5" x 5" sheet with 4 color filters costs $6 plus shipping and handling.2. Tom Donohue and Howard Wallace, "Ultraviolet Viewer," Physics Teacher 31, 41

(1993).3. Science Kit, P.O. Box 5059, San Luis Obispo, CA 93403, (800) 828-9572.4. R. S. Halada, "Demonstrations of Infrared Ray Optics Using Ray Guns," Physics Teacher

29, 370 (1991).5. John W. Jewett, Jr., "Physics Begins With An M,” p 311, Allyn and Bacon (1994).6. The IR absorbing filter $22.50 and the IR bandpass filter $52.80 are available from

CENCO Scientific, 3300 CENCO Pkwy, Franklin Park, IL 60131, (800) 262-3626. Theradiometer is a common device available from most science supply houses (including theReuben H. Fleet Space Museum) for about $5–$10.

7. Ron Fritz, "Calibration and Efficiency of Microwave Ovens,",Physics Teacherr 28, 564(1990).

8. John W. Jewett, Jr., "Physics Begins With An M,” p 323, Allyn and Bacon (1994).

The Electromagnetic SpectrumIntroduction–Invisible Regions

13

Laboratory No. 2:

Demonstrating the Invisible Regions of theElectromagnetic Spectrum

Purpose

The purpose of this demonstration is to investigate the invisible electromagnetic spectrum byemploying various detectors to indicate the presence of waves.

Required Equipment and Supplies

Diffraction grating; variable voltage supply; 100–200 W clear lamp and socket; sensitivetemperature sensor; UV source and detector; IR source and detector; small portable radio orTV; Faraday cage, metal box, or wire mesh box; microwave oven; distilled water; NaCl;two 250 ml beakers; stopwatch; leaf electroscope; Crookes tube and induction coil; electricalwire; gamma source; aluminum sheet; lead sheet.

Discussion

Energy that is emitted from vibrating electric charges produces electromagnetic waves.

Power waves, radio/TV waves, and microwaves are produced by atoms or molecules

vibrating slowly over macroscopic distances. Infrared waves are produced by more rapidly

vibrating atoms or molecules or by slowly vibrating electrons. Electrons vibrating at a faster

rate produce visible light. Even more rapidly vibrating electrons produce ultraviolet and X-

rays. Gamma rays are produced by nuclear transitions (changes of the nucleus from one

energy level to another). It can generally be stated that the more massive the particle, the

more slowly it vibrates. Therefore only tiny masses, such as electrons, can vibrate fast

enough to produce high frequencies, whereas large masses, such as atoms and molecules,

vibrate slowly enough to produce low frequencies. The faster an object vibrates, the more

energy it can release.

Demonstration

Name: Class: Date:

14 Demonstrating the Invisible Regions Laboratory No. 2

14

Review Questions1. Make a statement that relates the mass of a particle to the kind of wave it produces.2. What causes an electromagnetic wave?3. What is a heat wave? Can you see a heat wave?4. Why can't you see UV or IR?5. What is a nuclear transition?

ProcedureRecord your observations for each of the types of invisible radiation that your teacherdemonstrates.

Analysis of ExperimentAnswer the following question based on your observations in class:

1. Blackbody Radiator(a) What do you notice about the spectrum as the brightness (temperature) of the light

bulb increases?(b) What proof do you have from the demonstration that electromagnetic waves are

produced by a heated but not glowing bulb?

2. Ultraviolet(a) Describe the UV detector.(b) What sources of UV did you detect?

3. Infrared(a) Describe the IR detector.(b) How did this investigation demonstrate the presence of IR waves?

4. Radio/TV(a) How does the shielding experiment demonstrate that radio signals are waves?

5. Microwaves(a) Why don't microwaves pass through the screening on the oven door?(b) Why does salted water heat faster than pure water in the microwave oven?

The Electromagnetic SpectrumSunlight

15

Instructor’s Guide to Lab No. 3 (a & b):

Why Are There Colorsin a Compact Disk?

GoalThe goal is to introduce the phenomena of interference and diffraction using a commoneveryday items such as a audio compact disk (CD). Identify natural examples of iridescencefound in nature.

ObjectivesAfter performing this exercise students will be able to:

• Understand the phenomena of wave diffraction• Understand the phenomena of wave interference• Use a diffraction grating to separate a visible light source into its component parts.• Calculate the wavelength of different colors of light using an ordinary CD


We have divided this lab into two sections: the first just examines the cause of the colorsin the CD and the general property of iridescence and interference patterns. The second partactually uses the CD as a diffraction grating; students start with the known value of thewavelength of violet light (450 nm) and then determine the spacing between tracks on theCD.

This lab offers a fascinating look at the CD as a diffraction grating. An excellentdescription of the phenomena is presented in the student labs. Because of their wide use anddesirability, students find it quite compelling to learn about the CDs. This might be a goodopportunity to introduce some of physics that goes on in the standard operation of a CDplayer. The article by T.D. Rossing is a fitting reference here.

It is interesting to note that there are no continuous grooves or even tracks present as amechanical structure on a CD. The occurrence of closely spaced pits, however, is sufficientto give the strong visual interference effects. This shows that an ideal grating is not required;a sufficiently periodic structure also does the job!

A similar phenomena is thin film interference. See the demo described in the extensionbelow. The bright colors in a peacock’s feathers, as well as the similarly bright colors on thethroat of a hummingbird, are due to interference, not to absorption and reflection as with“normal” colored objects. Structures in the feathers act as multilayer interference films thatexhibit constructive interference for the colors that you see from the feathers, for example,blue and green from the peacock feather. Look at the peacock feather from different anglesand notice how the color changes. Other creatures exhibit similar interference effects, such asthe Morpho butterfly from South America and the beetle Chrysochroa fulminans, for whichthe interference combines with a highly glossy surface to give colors which range frommetallic gold to green (Jewitt, 1994).

Activity

16 Instructor’s Guide to Lab No. 3 (a & b)

16

Helpful Hints• Use a peacock feather for students to examine natural examples of iridescence. Have

students look at the peacock feather from different angles and notice how the colorchanges. Introduce them to abalone shells and have them identify the multiple thin layersthat cause the beautiful colors.

• Set up various light sources for the students to study with the CD as a diffraction grating.Use the mercury and neon lamps included in the resource box.

• Sample data for the track spacing measurements in Lab No. 3b is: s = 20 cm, r = 6.0 cm,which results in a track spacing of d = 1566 nm, very close to 1600 nm!

Extensions• Another way to do this experiment would be to start the students with the known track

spacing for the CD (1600 nm from manufacturers data) and have them determine theapproximate wavelength of violet light (450 nm).

• After measuring the track spacing, you may want to have you students approximate thetotal track length on the CD. A good way to do this would be measure the width of theCD surface (r router inner− ) and divide by the track spacing (1600 nm) to determine the totalnumber of tracks. Then multiply this by the average radius 1

2 ( )r router inner+ . This shouldcome out to be several miles!

• Do your students believe that light can never pass through a metal, no matter howeverthin it is? Make them look through a CD. They have to believe you, though, that thematerial inside is a metal.

• Interference by Thin Films - Dip a dark-colored coffee cup (dark colors make the bestbackground for viewing interference colors) in dish washing detergent, and then hold itsideways and look at the reflected light from the soap film that covers its mouth. Swirlingcolors appear as the soap runs down to form a wedge that grows thicker at the bottomwith time. The top becomes thinner, so thin that it appears black. This tell us that itsthickness is less than one-fourth the thickness of the shortest waves of visible light.Whatever its wavelength, light reflecting from the inner surface reverses phase, rejoinslight reflecting from the inner surface reverses phase, rejoins light reflecting from theouter surface, and cancels. The film soon becomes so thin it pops.

References

J.W. Jewitt, Physics Begins With an M (Allyn and Baker, Needahm Heights, MA, 1994).

T.D. Rossing, “The Compact Disc Digital Audio System,” The Physics Teacher 25, 556(1987).

C. Noldeke “Compact Disk Diffraction,” The Physics Teacher 28, 484 (1990).

G. Ramme, “Colors on Soap Films – An Interference Phenomenon,” The Physics Teacher28, 479 (1990).

The Electromagnetic SpectrumVisible Light

17

Laboratory No. 3a

Why Are There Colorsin a Compact Disk?

PurposeTo investigate the diffraction and interference of light reflected from a normal audio compact disk.

Required Equipment and SuppliesCompact disk (CD), ordinary incandescent light source, miscellaneous light sources.

DiscussionThe rainbow of colors reflected from

the surface of a compact audio disk is afamiliar sight. This is the same display ofcolors that is produced by a diffractiongrating such as the one used in theprevious labs to investigate visible spectra.This means that a CD essentially behavesas a diffraction grating. In order to understand how a diffraction grating works to separate colors oflight, we need to examine some of the special wave properties of light. The wave properties of lightcreate some of nature’s most beautiful spectacles, including the colors in a peacocks tail, abaloneshells, rainbows, and soap films.

One of the most interesting properties of waves is calledinterference, which is caused by the overlapping of wavessharing the same space at the same time. When waves overlapthey combine to form a new wave which is the sum of theeffects of each wave. Interference can be either constructive ordestructive. Figure 2 shows a hypothetical situation in whichwaves from a storm off the Alaska coast might interfere withwaves from Hawaii near a California beach. When the wavesfrom each storm arrive crest-to-crest (“in-phase”) they interfere constructively and combine to form astronger wave. When the waves arrive crest-to-trough (“out of phase”) they interfere destructively andcancel one another out. Interference effects can be either partial or complete.

When light is reflected from a regular pattern of tiny objects, interference causes colors to appear.Figure 1 shows how light striking the reflective surface of a CD composed of regularly spaced trackscan interfere constructively, causing intense reflection of particular wavelength at certain angles. If theviewer changes angles with respect to the CD, some other wavelength interferes constructively – thecolor seen depends on the angle of observation, just as with a rainbow.

Such colors from interference are called iridescence (iris: Latin for rainbow). The shells and wingsof some wasps and beetles have parallel grooves that produce iridescence. Iridescent butterflies havescales that act as reflective gratings. Iridescence can also come from constructive reflections of thinfilms such as soap films or gasoline on a wet street. The brilliant iridescent blues and greens fromsome types of seaweed and from abalone shells come from constructive reflections from multiple thinlayers. The bright colors in a peacock’s feathers and the throat of a hummingbird are also due toiridescence. Structures in the feathers act as multilayer interference films that exhibit constructiveinterference for the colors that you see from the feathers.

Experiment

Name: Class: Date:

∆λ violet

∆λred

Incoming Light

Diffracted Light 67

8

678

Figure 1B

Constructive Interference(Complete)

Destructive Interference(Complete)

+

=

+

Wave offCalifornia

Coast

AlaskaStormWave

HawaiiStormWave

Figure 2

18 Why Are There Colors in a Compact Disk? Laboratory No. 3a

18

As well as being beautiful, interference patterns are extremely useful, and provided the firstconvincing demonstration of the wave nature of light. We can use a reflective surface with regularlyspaced grooves in it, called a diffraction grating, to measure and study light. In the previous labs youused another type of grating called a transmission diffraction grating to disperse light into itsconstituent colors and to determine the colors of light that are emitted by different light sources.

Both transmission and reflection-type diffraction gratings are extensively used in the sciences tostudy light; the use and function of each is essentially the same. Both types are manufactured verycarefully, and typically contain six-hundred or so grooves or lines per millimeter!

It turns out that the regular pattern of pits contained on the reflective surface of a CD causes theCD to behave much like a grating. As with a manufactured grating, different angles of viewing causeconstructive interference to occur for different wavelengths (colors) of light. Thus, reflection of whitelight off the surface gives a spectrum of colors across the surface of the CD. At small viewing angles,the shorter wavelengths constructively interfere (violet, blue, and indigo) while the longer wavelengthswill constructively interfere at larger viewing angles (red, orange, and yellow). Interestingly enough,diffraction gratings might be more reasonably called interference gratings!

Review Questions1. What is interference?2. What is iridescence?3. How is a CD similar to a diffraction grating?4 In what way would a diffraction grating be better called an interference grating.

Activities

1. Take a normal audio CD to a incandescent source of light and examine the interference patternthat results from reflected light. What colors do you see? What order are they in? Which end of thespectrum is closest to you, violet or red? What does this say about the wavelengths of each color inthe spectrum?

2. Examine the CD in front of the mercury light source. What colors do you see? What does this sayabout the wavelengths of light emitted by excited mercury gas. Use the CD to examine the lightemission of other light sources setup by your instructor, such as a neon glow lamp, Gro-Lux lamp,and Cyalume light sticks.

3. Are the brilliant feathers of a peacock really blue and green? Look at a peacock feather at differentangles and notice how the color changes. What is going on here? What is the source of the brilliantblues and greens we see? Pigmentation or interference effects?

4. You can do this one as a home project in the kitchen sink. Dip a dark-colored coffee cup (darkcolors make the best background for viewing interference colors) in dish washing detergent, andthen hold it sideways and look at the reflected light from the soap film that covers its mouth.Swirling colors appear as the soap runs down to form a wedge that grows thicker at the bottomwith time. This is called thin-film interference. Ask your instructor for a detailed description.Interference phenomena is all around us! Can you spot more?


19

Laboratory No. 3bThe Compact Disk as

Diffraction GratingPurposeTo use an audio compact disk (CD) as a diffraction grating and the known wavelength of violet lightto measure the spacing between tracks.

Required Equipment and SuppliesCompact disk, ordinary incandescent light source (40 W bulb will do), and a ruler.

DiscussionWhen light is reflected from a regular pattern of tiny objects,

interference causes colors to appear. In the previous lab we saw howlight striking the reflective surface of a CD composed of regularlyspaced tracks can interfere constructively, causing intense reflectionof particular wavelength at certain angles. The pattern of colorsgenerated by both reflection and transmission diffraction gratings iscalled an interference pattern. These patterns can be both strikinglybeautiful and extremely useful, and provided the first convincingdemonstration of the wave nature of light.

Both transmission and reflection-type diffraction gratings areextensively used in the sciences to study light; the use and function ofeach is essentially the same. Although the interference patternsproduced by gratings are generated by the interference properties oflight, there is another important wave phenomena going on here called diffraction. This is one of themore obvious properties of waves and refers to the spreading and bending of waves around objects.This is why sound can bend around corners, allowing you to hear a stereo play from another roombefore you actually enter the room and see the stereo. You might wonder, though, why light does notbend in the same manner, if it is truly a wave. If light and sound are both waves, why don’t they actthe same? The reason turns out to be one of size. In fact, light does bend, but on a much smaller scalebecause of its much smaller wavelength. In order for diffraction to be noticeable, the object causingthe bending or spreading must be about the same size as the wave. This is only several hundrednanometers (10-9 m) for visible light! This is why typical diffraction gratings used for visible containthousands of closely spaces lines (called rules or slits), about 600 per millimeter.

When light from a light source passes through a transmission diffraction grating, each slit in thegrating diffracts or spreads the light as if it were originating from a point source. In effect, adiffraction grating produces thousands of closely-spaced mini-light sources. Furthermore, since thelight originated from the same source behind the grating, the light from each slit is coherent(synchronized) with one another. A compact disk, with its closely spaced grooves and reflectivealuminum plating, essentially provides thousands of tiny, closely spaced mirrors capable ofdiffracting light by reflection. It is these closely spaced coherent light “sources” created by diffractionthat interact to produce the interference patterns that we use to detect, study, and measure light. Forexample, if we know the distance d between slits or rules on the grating, then we can use the gratingto measure the wavelengths of light emitted by any source using the grating. Conversely, we can use aknown wavelength of a particular color of light, such as violet (λ = 450 nm), to measure the tinydistance between tracks on a CD! In a sense, we’re using the wavelength of light itself as a sort ofsuper-fine meterstick to measure distances smaller the width of a human hair!

Now that we understand the concepts involved, let’s take a closer look at how we can use a CD asdiffraction grating to measure the tiny spacing between tracks. Although interference patterns mightseem complicated, it’s really just a matter of geometry and the simple fact that constructive

Experiment

Name: Class: Date:

IncomingWaves

Bright

Bright

Bright

Dark

Dark

Dark

DarkDiffracted

Waves

Grating

Screen

Figure 1

20 Why Are There Colors in a Compact Disk? Laboratory No. 3a

20

interference occurs at places that are multiple wavelengths from each source. The details of thecalculation will become clearer as you follow the procedure below.

Review Questions1. How is a CD similar to a diffraction grating?2. What is diffraction?3. What is interference?

Procedure1. Working with a partner, take a CD and a ruler and

stand about 2 m from the light source, whichshould be placed at eye level behind you. Hold the CD at various positions in the light andexamine the brilliant colors that are reflected from the surface.

2. Hold the CD in front of you so that the reflection of the bulb disappears in the center hole. Holdthe CD about 10 cm from your eye so that circular spectrum can be observed on the disk. Increasethe distance until a violet pat of the spectrum appears on the edge of the CD. Measure thedistance between your eye and the disk: this is the distance s in Figure 2. With the help of thevisual structures on the edge of the CD, take notice of the radial position of the violet ringappearing on the disk. Measure the radius of the disk from the center to outside edge where theviolet ring was observed; this is the distance r in Figure 2.

AnalysisA typical drawing detailing the geometry for constructive interference

from a diffraction grating is shown in Figure 3. The grating constant d iseffectively the track spacing of the CD. Using the accepted value of λ = 450nm for violet light, use this drawing together with your measured values of rand s to determine the approximate track spacing d. The procedure isoutlined below.

For a typical diffraction grating, light rays that are reflected straight backwithout deviation interfere constructively to produce the brightest image atthe center of the screen (or the observer’s eye). This image is useful foralignment of the diffracted spectra, and was conveniently passed through thehole of CD in this experiment. Constructive interference also occurs for anyangle θ such that the rays from adjacent tracks each travel an extra distanceof ∆l n= λ , where is n is an integer that denotes the order of the image.Using trigonometry, we see that constructive interference occurs when theangle θn is such that

∆l d n nn= = =sin , , ,...θ λ 1 2 3

The ring you measured on the CD is the first in a series of spectral images, and is called the firstorder (n = 1). The angle θ in Figure 3 is the same as the angle shown in Figure 2. By varying thedistance s you effectively changed the angle θ, which causes different wavelengths (colors) of light toconstructively interfere. We can use the Pythagorean theorem for right triangles to calculate the sineof the angle θ:

sinθn

r

s r= =

+opposite

hypotenuse 2 2

The track spacing d can be found by combining these two the equations and rearranging a little:

d n ns r

rn

= = ⋅ + = ⋅( ) + ( )

=λθ

λ11 450

2 2 2 2

sin( )(

(nm)

cm cm

cm)nm

The international manufacturing standard for the track spacing of a CD is 1600 nm. How does yourmeasured value compare with this standard?

Eye

r

2 m s

Lamp CD

θ

Figure 2

θ

θ

∆l=n•λ

d

Incoming Wavefronts

Outgoing Wavefronts

Figure 3


21

Instructor’s Guide to Laboratory No. 4:

Measuring Wavelengths of LightGoalThe goal is to learn how to measure the wavelengths of various colors of light and linespectra using an optical slit with a diffraction grating and simple optical bench.


• Understand the phenomena of wave diffraction• Understand the phenomena of wave interference• Use a diffraction grating to separate a visible light source into its component parts.• Calculate the wavelength of different colors of light using an ordinary CD


An excellent description of the phenomena is presented in the student lab. Everythingrequired to do this experiment except a meterstick is included in the Resource Box.

This experiment can also be done as a demonstration using the TV camera as theobserver. Clear line spectra can be seen on the screen. This setup can then be used with an IRsource to view IR spectra.

Helpful Hints• You may want to introduce the interference patterns here with the Moiré-pattern slides

included in the Resource Box. Use an OHP and overlay the two slides on one another.Move the slides around to demonstrate various configurations of interference. You maywant to save this demo for the next lab, Young’s Experiment.

• An excellent way to conduct this lab is to assign the measurement of different linespectra to different groups of students.

• The mercury source included in the Box has several filters to isolate several differentmercury lines.

• An incandescent-type neon bulb has some rich lines that can be observed.• You may want to do the Young’s Experiment lab before this one.

Extensions• This setup can then be used with an IR source to view IR spectra.• This experiment can also be done as a demonstration using the TV camera as the

observer. Clear line spectra can be seen on the screen. This setup can then be used withan IR source to view IR spectra.

References

J.D. Wilson, Physics Laboratory Experiments 3e (D.C. Heath and Company, Lexington, MA,1990), Chap 50.

Experiment


22


23

Laboratory No. 4Measuring Wavelengths of Light

PurposeTo measure the wavelengths of light using a diffraction grating.

Required Equipment and SuppliesOptical bench (meterstick with supports), optical slit with scale and holder, diffractiongrating with holder, mercury light source, incandescent light source.

DiscussionA diffraction grating consists of a piece of

metal or glass with a very large number ofevenly spaced parallel lines or grooves.Common laboratory gratings have 200 or 600groves per mm. There are two types ofgratings: reflection gratings and transmissiongratings. Reflection gratings are ruled onpolished metal surfaces and light is reflectedfrom the unruled areas which act as a row of“slits.” Transmission gratings are ruled on glass and the unruled slit areas transmit incidentlight. The transmission type diffraction grating is used in this experiment.

Diffraction refers to the “bending” of waves around sharp edges or corners. The slits of agrating cause light to be diffracted, and the diffracted light interferes with itself so as to setup interference patterns, which produces a series of images of the source slit (Figure 1). Thebrightest image is the undeviated and undiffracted central maximum, which appears directlyin front of the slit as expected. Complete constructive interference of the waves occurs wherethe phase or path difference is equal to one wavelength, which occurs symmetrically on bothsides of the central maximum at locations corresponding to

d n nnsin , , ,...θ λ= = 1 2 3

where λ is the wavelength of light being diffracted, n is the order of the image being formed(first, second, etc.), d is the grating constant or the distance between the grating lines, θn isthe angle the rays are diffracted from the incident direction, and d nsinθ is the path differencebetween adjacent rays. The grating constant is given by

d N= 1/

where N is the number of lines or grooves per mm of the grating.

These devices, like prisms, disperse white light into colors. Whereas a prism separates thecolors of light by refraction, a diffraction grating separates colors by interference. Usuallyonly the first few orders are easily observed, with the total number of orders depending onthe grating constant. If the incident light is monochromatic (composed of a singlewavelength), the grating will spread the light into a series of well-determined lines. Thewavelength of these lines can be determined with a simple optical bench.

Review Questions1. What is a diffraction grating?2. What is diffraction?3. How is a diffraction grating similar to a refraction prism?

Experiment

Name: Class: Date:

SourceSlit

Grating

Central maximum(n = 0)

n = 2

Second ordern = 2

n = 1

First ordern = 1

θ1

θ2

24 Measuring the Wavelength of Light Laboratory No. 4

24

Procedure1. Record the number of lines per mm of your

grating in part in the data table below. Mountthe grating and the slit scale on themeterstick-type optical bench as shown inFigure 2. The planes of the grating and theslit scale should be parallel.

2. Position an incandescent light source behindthe slit and observe the diffraction orders ofthe continuous spectrum superimposed onthe scale with distance s between the slit andthe grating at 60, 80 and 100 cm. Lookingthrough the grating, note the difference in the pattern positions x1 and x2 for the first twoorders in each case. The images of the slit for a given order should appear at equaldistances from the center line. If they do not, rotate the grating slightly until they do.

3. Replace the incandescent light with the mercury vapor lamp fitted with one of the colorfilters. Record the color of the filter in the data table below.

4. Looking through the grating, measure the apparent displacements of the brightest line inthe first or second order spectrum, for both the left and right sides. Record yourmeasurements for s = 60, 80, and 100 cm in the data table, and the order n you chose tomeasure.

5. From Figure 2 it can be seen that sinθn for a given order can be determined usingtrigonometry, that is

sinθ θ= =+

side opposite hypotenuse

x

s x2 2

Compute sin for the measured first and second order for each distance s and find theaverage value of sinθn for each order.

6. Compute the grating constant d. Convert this number to nanometers by multiplying by106 (1 mm 10 nm6= )and record in the tablebelow. Calculate theaverage wavelength ofthe measured mercuryline using the equationd nnsinθ λ= . Be sureto include the correctorder n you chose tomeasure.

Analysis of Experiment

Fill out the data table andcalculate the wavelength ofthe measured line. Find outfrom your teacher what theaccepted value is. Can youidentify any sources of errorin your measurements?

Source

Observer

Grating

Left FirstOrder Image

s

x1

x2

θ1θ2

Right SecondOrder ImageSlit

Metric Scale

Data Table

Grating lines per mm:

N =

Image order:

n =

Color

Distances (cm)

xn left

(cm)

xn right

(cm)

Avg. xn

(cm) sinθn

Average sinθn

Grating constant dN

= ×( ) ( )1

106 mm nm/mm nm

Wavelength λ θexp sin( )

( )( )= =d

n n

nmnm


25


Young’s ExperimentGoalThe goal is to reproduce Young’s famous double-slit experiment, verify the wave nature oflight, and measure the wavelength of red light.


• Understand the phenomena of wave diffraction.• Understand the phenomena of wave interference.• Understand the method and phenomena of double-slit interference.• Reproduce Young’s classic experiment and verify the wave nature of light


An excellent description of the method and history of this experiment is presented in thestudent lab. Everything required to do this experiment except a meterstick is included in theResource Box.

This experiment is even more dramatic when done with a laser. The pattern may beprojected on a screen.

Helpful Hints• Introduce the interference patterns here with the Moiré-pattern slides included in the

Resource Box. Use an OHP and overlay the two slides on one another. Move the slidesaround to demonstrate various configurations of interference. You may want to save thisdemo for the next lab, Young’s Experiment.

• This experiment is even more dramatic when done with a laser. The pattern may beprojected on a screen.

Extensions• You may want to duplicate Young’s original method of manufacturing a double slit: hold

a microscope slide inverted over a candle and carefully coat the surface evenly withlampblack. Scratch two slits as per the student lab.

• Thomas Young made many contribution to various fields. He makes a good biographicalsubject.

ReferencesConsult any good Physics text for a detailed history and explanation of this experiment.

Experiment


26


27

Laboratory No. 5Young’s Experiment

PurposeTo reproduce Young’s double-slit experiment and measure the wavelength of red light.

Required Equipment and SuppliesOptical slits kit, ruler, masking tape, micrometer (optional).

DiscussionIn 1801 the wave nature of light was convincingly demonstrated when

the British physicist and physician Thomas Young performed his now-famous interference experiment. Young found that light directed throughtwo closely spaced pinholes recombined to produce fringes of brightnessand darkness on a screen behind. The bright fringes resulted from lightwaves of the two holes arriving crest to crest, while the dark areasresulted from light waves arriving trough to crest. This pattern of interference fringes is called an interferencepattern, and is a general wave phenomena that arises whenever a series of waves arrive at the same place fromtwo synchronized sources, or from the same source by traversing two different paths.

The easiest way to demonstrate an interference pattern is with sound waves from two synchronized speakers,each sounding the same signal. Because of the wave nature of sound some surprising effects occur: the totalloudness is not simply double that which would occur from a single speaker! Looking at the figure above, wesee that at the speakers, both sound waves are perfectly in step. But most places in the room in front of them arecloser to one speaker than to the other, so the waves don’t arrive perfectly synchronized since they havetraveled different distances to reach their common destination. Point A in the figure is exactly one wavelengthfarther from the right speaker than from the left one, and so arrive exactly one wavelength out of step. Theinterference between the waves here is constructive, meaning the waves reinforce each other and produce anextra strong tone. The same condition applies at point B, which is equally as far from the left speaker as fromthe right one.

Point C, however, is one-half wavelength closer to its nearest speaker, and here the waves arrive exactly outof step. The maximum air pressure for one wave coincides exactly with minimum air pressure for the other. Inthis case we get destructive interference, meaning the waves cancel each other and little or no sound is heard!

The key to understanding an interference pattern is straightforward: take the difference between thedistances from the two sources and divide by the wavelength. The resulting number will tell you what kind ofinterference will take place. If it is an integer (i.e., 0, 1, 2, 3, ...), the interference is constructive. If it lieshalfway between two integers (i.e., 1

2 ,1 12 , 2 1

2 , ...), the interference is destructive. Intermediate values will giveintermediate results, including not-quite-perfect reinforcement and not-quite-perfect cancellation; within aninterference pattern, wave effects may be increased, decreased, or neutralized.

It is hard to synchronize two light sources, so interference patterns with light are usually produced bysplitting a light beam into two or more parts and recombining them on a screen. This was originally done byThomas Young using two closely spaced pinholes; each tiny pinhole behaved as a synchronized source becauseof another wave phenomena known as diffraction. Diffraction is the bending of waves around sharp objects,which causes waves to spread out as if originating from a point source.

Young’s experiment is now done with two closely spaced slits instead of pinholes, so that the fringe patternsare straight lines. You can observe the interference of a single-slit diffraction pattern by holding up your hand toa light source with two fingers closely spaced together. The light passing through the “slit” between yourfingers is seen as a series of lines! Interference of light waves does not, by the way, create or destroy lightenergy; it merely redistributes it.

Interference patterns are not limited to single and double slits. A multitude of closely spaces slits make up adiffraction grating. These devices, like prisms, disperse white light into colors. Whereas a prism separates thecolors of light by refraction, a diffraction grating separates colors by interference.

Review Questions1. How was the wave nature of light demonstrated?2. What is diffraction?3. What is interference?

Experiment

Name: Class: Date:

A

BC

L R

28 Young’s Experiment Laboratory No. 5

28

Procedure1. Coat a glass slide with a colloidal suspension of graphite and let it dry. Be sure the coating is

uniform. Scratch a pair of slits as shown in the sketch. Hold the two razor blades tightly togetherand use little pressure. Make several pairs of slits. Select for use those which show at least threeclear white lines when you look a the line filament lamp. Scratch a window across each pair ofslits as shown.

2. Tape a clear slide over the graphite surface to protect the surface. The width between the slits isequal to the thickness of one razor blade. If available, use a micrometer to determine the thicknessof a single blade, or else use an ordinary ruler to measure the thickness of a stack of blades anddivide by the number of blades. Record the thickness d in the data table below.

3. Connect the lamp to 115V. Use a ringstand to mount a ruler slightly above the lamp. Lookthrough the slits toward the filament of the light bulb from a distance of about 2 meters (Figure2). Note what you see.

4. Tape two paper markers at positions on the ruler about where the farthest dark fringes (nodallines) can be seen. Since the nodal lines come in symmetric pairs (one on each side of the center),you will use these markers to measure the distance between the farthest pair of nodal lines youcan observe.

5. Cover part of the bulb with red cellophane (using an elastic band) and note the effect on thepattern. The interference pattern and the paper markers on the ruler can be seen simultaneously bylooking through the slits and the “window” scratched in the slide at the same time.

6. Now cover the whole bulb with red cellophane. Looking through the slide, move toward or awayfrom the ruler until you can align the furthest visible pair of fringe lines with the paper markerson the ruler. Determine which number nodal line you are aligning to by counting the total fringesbetween the markers and dividing by two. Also record the distance from the double-slit to theruler.

7. Now cover part of the bulb and part with blue. Note from your observations which color youthink has the shortest wavelength, and estimate the ratio of the wavelength of red light to thewavelength of blue light.

Analysis of ExperimentYoung determined the double-slit interferencepattern obeys the mathematical relationship

d n nnsin , , ,...θ λ= −( ) =12 1 2 3

You can use this with your experimentalmeasurements to calculate the wavelength of light!To a good approximation sin /θn nx s≈ .

Using the data table to the right, calculate

λ θ=−( )

d

nnsin

12

=

Two razor bladesheld tightlytogether

Coatedslide

θn

2x

nodal lines lamp

double slit

s

Data Table

Distance between slits d mm

Convert to nm ( d ⋅106) nm

Nodal line measured: n =

Distance between nodes cm

Half of distance ( xn ) cm

Distance from ruler s cm

sin /θn nx s≈

Est. Ratio: λ λred blue/ cm


29


Blue Skies and Red SunsetsGoalThe goal here is to investigate the processes of scattering and absorption which give rise tothe different colors of the sky, sunsets, clouds, and oceans.

ObjectivesAfter performing this activity students will be able to:

• Understand the process of light scattering by small particles and molecules.• Duplicate the effect using a flashlight and bowl of water.• Apply their new knowledge of color theory• Understand why the sky appears blue.• Understand why the sun appears to become increasingly redder as it sets.• Describe the different scattering effects that make clouds appear blue.• Explain why oceans appear blue.


This activity is fully described in the student handout. You may wish to have students dothis one at home.

It is interesting to note that the blue of the sky varies in different places under differentconditions. A principal factor is the water vapor content of the atmosphere. On clear dry daysthe sky is a much deeper blue than on clear days with high humidity. Places where the upperair is exceptionally dry, such as Italy and Greece, have beautifully blue skies that haveinspired painters for centuries. Where there are a lot of particles of dust and other particleslarger than oxygen and nitrogen molecules, the lower frequencies of light are scattered more.This makes the sky less blue, and it takes on a whitish appearance. After a heavy rainstormwhen the particles have been washed away, the sky becomes a deeper blue.

Helpful Hints• You may want to do this one as a demo using a bright light source such as a slide

projector.• This lab makes a good home project, since it only needs a flashlight and a large glass

bowl.

Extensions• Have students investigate the polarization of the scattered light using Polaroid filters

included in the Resource Box. Have them compare this to their findings for the real sky(the one outdoors!)

ReferencesConsult any good Physics or Physical Sciences text for a detailed description of this andrelated phenomena.

Activity


30


31

Laboratory No. 6

Blue Skies and Red SunsetsPurposeTo investigate how the scattering of sunlight by the Earth’s atmosphere produces blue skiesand orange sunsets.

Required Equipment and SuppliesFlashlight or slide projector, large glass bowl or pitcher filled with water, a few drops of milkor a pinch of coffee creamer, and a polarizing filter (optional).

DiscussionWhen light interacts with objects that are much smaller than the wavelength of the light,

the light is said to be scattered rather than reflected. The electrons of such a small object areall shaken up and down at the same time by the electric field of the light wave, and theyradiate that frequency of light in all directions. It turns out that the higher the frequency ofthe light, the more the light is scattered.

The diameter of most molecules is much smaller than the wavelengths of visible light.Most of the ultraviolet light from the sun is absorbed by a thin protective layer of ozone gasin the upper atmosphere, and the remaining ultraviolet sunlight that passes through theatmosphere is scattered by atmospheric particles and molecules. Of the visible frequencies oflight, the high-frequency violet is scattered the most, followed by blue, green, yellow,orange, and red, in the order of decreasing frequency. Red is scattered less than a tenth asmuch as violet. Although violet light is scattered more than blue, our eyes are not verysensitive to violet and there tends to be more blue light in sunlight than violet. The bluepredominates in our vision, so we see a blue sky!

The grayish haze in the skies of large cities is a result of particles emitted by internalcombustion engines (cars, trucks, industrial plants). Even when idling, a typical automobileengine emits more than 100 billion particles per second. Most are invisible and provide aframework to which other particle adhere. These are the primary scatterers of lowerfrequency light. For the larger of these particles, absorption rather than scattering takes placeand brownish haze we call smog is produced. Yuk!

Since the lower frequencies of light are scattered the least by nitrogen and oxygenmolecules (the primary components of our atmosphere), red, orange, green, and yellow lightare transmitted through the atmosphere much more than violet and blue. Red, which isscattered the least, passes through more atmosphere than any other color. Therefore, whenwhite light passes through a thick atmosphere, the higher frequency blue and violet isscattered the most while the lower frequencies such as red are transmitted with minimalscattering. Such a thicker atmosphere is presented to sunlight at sunset, since the paththrough the atmosphere is longer as the sun is lower on the horizon. This means that the sunbecomes progressively redder as the sun goes down, going from yellow to orange.

Clusters of water molecules in variety of sizes make up clouds. The different size clustersresult in a variety of scattered frequencies: the tiniest, blue; slightly large clusters, green; andstill larger clusters, red. The overall result is a white cloud! For even larger droplets,absorption occurs and the scattered intensity is less. The clouds are darker. What about evenbigger drops? Well, their increased size causes them to fall to earth, and we have rain!

Activity

Name: Class: Date:

Blue Skies and Red Sunsets Laboratory No. 6

32

34

Since we’re on the subject of colors, let’s discuss water. The color of water is not thebeautiful deep blue that you often see on a surface of a lake or the ocean. That blue is thereflected color of the sky. The color of water itself, as you can see by looking at a piece ofwhite material under water, is a pale greenish blue.

Although water is transparent to nearly all the visible frequencies of light, watermolecules very weakly absorb visible red light, and strongly absorb infrared waves. Theenergy of infrared waves is transformed into internal energy in the water, which is whysunlight warms water. Weakly-absorbed visible red light is reduced to a quarter of its initialbrightness by 15 meters of water, and there is very little red light in the sunlight thatpenetrates below 30 meters of water. When red is taken away from white light, what colorremains? This question can be asked another way: What is the complementary color of red?The complementary color of red is cyan – a bluish green color. In sea water, the color ofeverything at these depths looks greenish.

So while the sky is blue because blue is strongly scattered by molecules in theatmosphere, water is bluish green because red is absorbed by molecules in the water. We seethat the colors of things depend on which colors are scattered or reflected by molecules andalso on which colors are absorbed by molecules.

Review Questions1. What happens when light interacts with objects that are much smaller than the

wavelength of the light?2. Why does the sky appear blue?3. Why are sunsets red?4. What causes oceans to appear blue?

Activity1. Here’s a way to make your own blue

skies and reddish sunsets. When aflashlight beam penetrates a pitcher ofclear water, there’s little change in thecolor of the beam. Add a few drops ofmilk to the water, however, and you’llsee the beam of light turn a reddish orange (Figure 1). The milk’s molecules scatter theblue light (and some green and yellow, too) in all directions before it can reach your eyes,just as the air’s molecules do for the rays of sunlight at sunset. Now look through the sideof the pitcher, perpendicular to the beam. Wow! There’s the blue light scattered to thesides (and in all directions), just as the air scatters blue light from sunlight to give us blueskies.

2. Look at the scattered light through a polarizer. Rotate the polarizer and explain what yousee. Does this mean that the scattered blue light of the sky is polarized? After you answerthis question, take the polarizer outside or to a window and check your answer. Noticethe the polarization of different parts of the sky by rotating the polarizer.

Bluish light

Reddish light

A few drops of milkin a pitcher of water

Flashlight


33


Living on Borrowed SunshineGoal

The goal of this particular lesson is to allow students to use their own creativity in order tobetter understand and remember the complex pathways of photosynthesis.

ObjectivesAfter the explorations the students should be able to:

1. The student will understand the basic operation of photosynthesis by using creativewriting techniques to form a story that involves fundamental aspects of the process.

2. The student will appreciate the interrelatedness of light energy and energy transfers withlife on earth.

3. By using the story approach, the student will gain an appreciation for the complexity oflife processes such as photosynthesis.


The suggested lesson here is to have the students learn the process of photosynthesis by

writing a creative story. In this regard, the traditional, cool detachment of science is

disregarded and a more humanistic approach is used. By identifying photons, for example,

with human names, a student has an easier time remembering the complexities.

Photosynthesis can be broken down into two basic steps:

1. Light dependent reaction:a. water molecule breakdownb. photon capturec. coenzyme interactiond. photon capturee. coenzyme interactionf. NADPH+ reaction

2. Light independent reaction:a. carbon dioxide captureb. carbon fixing reactionc. introduction of hydrogensd. PGAL productione. cycle continuation

It would be helpful to set up stories or other activities so that they are limited to one of

the two major steps. This way other groups can interact with each other and exchange

information.

Additional Activities1. Using a traditional text as a source, create a comic or some other graphic outline to

"draw" out the story so it be used as an aid in understanding the plot. See attached.

Activity

34 Instructor’s Guide to Laboratory No. 7

34

2. Students often enjoy acting out the parts they discover in their story. A skit, allowingstudents to use their full range of talents, gives another avenue by which understandingcan be achieved. For example, students could be various co-enzymes, tennis balls couldbe used for photons, etc. An active process allows the students to grasp the meaning of anotherwise abstract concept. Besides, it's fun. It is recommended that a group of at leastfive students be used per skit. This allows the various portions of photosynthesis to bedisplayed without one student doing too many parts.

Helpful Hints• Note: This is a sample story to help the teacher better understand the potential of this

kind of assignment. It is suggested that it not be read or distributed to the students sincethey will have a tendency to use it instead of their own imagination. This particular storyfocuses on photons, but any angle is appropriate as long as it pulls in the major portionsof photosynthesis. It is assumed that the reader has some familiarity with the process.

Living on Borrowed SunshineIt was a typical beach party. Blankets on the sand, a roaring fire and marshmallows

roasting over the coals. It was here that I came upon a realization, an epiphany beyond mywildest dreams. In fact it took a dream to come full circle to help me understand what it allmeant. It has to do with the fire that resides within us all. The borrowed sunshine that powersall life on earth.

But first, the beach party.

"Your marshmallow is on fire," a friend warned.Pulling the burning puff of sugar from the flames, I wondered out loud why it was burning atall.

"Sugar," my friend said."Sugar?""Yes. All sugar will burn if you give it enough heat. Just like the wood in the fire."It was then I began to wonder. Burning sugar. Burning wood. Where did all this potential

fire come from? Then it hit me. The sun! Both the sugar and the wood had been produced byplants. The plants had taken energy from the sun and stored it within their roots, stems andleaves. The fire I was watching was actually borrowed sunshine being released into the air,warming everything around it.

"Fire. Sunshine. Photosynthesis. It all makes sense now," I mumbled to myself."What?"I turned to my friend and tried to explain. It wasn't long however, before his eyes began

to glaze over and roll like the cherries in a slot machine. I took this as a hint of inappropriateparty conversion and excused myself.

I left the party and walked alone down the beach, finding a soft spot to rest to furtherponder my thoughts. It was late and the day's activities had left me spent. Therefore, it wasn'tlong before I succumbed to my own heavy eyelids and wandering neurons. A pre-snoozingbody jerk eventually left me falling into a deep sleep. Thoughts of sunshine danced in myhead. The answers to my questions were to come to me in my dreams . . .

Polly the photon was wiggling her way to earth through the void of space. As a packet oflight energy generated by the vibrating electrons on the sun, she represented a distinctquantity of light. And as such, she could cause only certain kinds of reactions when she hitthe surface of some distant molecule. Like many of her other visible light friends, she mightvery well end up traveling through the windshield of some parked car, hitting the plastic seatsand be converted into basic heat energy by the molecules she bumped into. Unable to escape

Living on Borrowed Sunshine 35

35

the car's interior due the new, lower energy level, and hence larger wavelength, her spentenergy would help turn the passenger compartment into a solar powered oven.

She would never have the opportunity to contribute to greater things.However, this was not to be Polly's fate. Her destiny would be much more productive by

her chance encounter with a leaf. For leaves need photons like Polly. Her wavelength andtherefore, her color, were just right for the energy requirements of photosynthesis, thepowerhouse of life on earth. Instead of being wasted on a hot steering wheel, Polly wouldend up helping to build the foundation of all living things.

Meanwhile, the leaves down on earth were waving in the wind, hoping for a few millionphotons to come their way. Not just any photon, but specific ones. The violet-blues and theorange-reds are desired most. These are what power the photosynthetic machine within theleaf. The greens, however, are shunned and reflected. Thus, the leaves appear green. Theyshow off the colors rejected, not absorbed. The colors we see are in fact the unwanted hues.In this sense then, the green leaf is every color but green. A confusing state indeed.

Polly had a wavelength that appeared red. 680 nanometers to be exact. A nanometer ispretty small. A million of them span the tiny distance of a millimeter. So Polly's wavelengthwas tiny, but compared to things like X-rays which can be a million times smaller and thusmore dangerous for their ability to penetrate into things, her waves were still quiterespectable. Not too small, not too large. This was exactly what the leaves were looking for.

Upon reaching the earth after leaving the sun little more than eight minutes before, at herstandard speed of 186,000 miles per second, Polly slammed into an apple tree leaf.

"Come on in," the chlorophyll molecule said with a snicker. "Welcome aboard."Along with another one of her friends, who was Polly's exact twin, the two photons

entered the photo system II station house. This was a magical place where the chlorophylllived and completely absorbed the energy of the appropriate incoming photons. Sadly, Pollyand her friend were no more, but their energy lived on. It was used to kick Mr. Z into actionand to push two tiny electrons through a series of molecules. Where, one may ask, do theselittle electrons come from? Surprisingly, the answer is water . Now the need for sprinklerscan be finally understood. Plants need water molecules to steal their electrons. Two electronsper molecule to be exact. This is accomplished within a little fellow called the Z particle. Mr.Z for short. Mr. Z beats up the water with energy from the photons, throws its two protonsinto the lumen, coughs out its oxygen and frees two new electrons to be energized by photonslike Polly and her friend. Once the electrons have ripped from their mother water, they travelthrough several stationary molecules. During the first part of their journey, their energy isused to pump protons into the lumen for the production of a special transfer molecule calledATP.

The lumen? This is a fluid filled space within a round, little structure called a thylakoid.Hundreds of these thylakoids lay in stacks, pancake stacks, within the chloroplast, a largerround object found by the thousands inside all green leaves. Little things within biggerthings. Such is the nature of nature. The leaf is no exception. And the H+ protons? These areatomic particles with a positive charge floating freely outside the lumen and becomingattracted to the moving, positively charged electrons.

I partially awaken myself with a snore as I snort in a lung full of air. "Ah, this is wheremy oxygen comes from", I thought to myself. "Coughing Z's, Mr. Z, Mr. Z, Mr. Z . . ." Idrift back to sleep and into my dreams.

But alas, energy cannot last forever. The energy given to them by Polly and friend isshortly exhausted. Time for more. At this point the two electrons hang out at the photosystem I station until two more photons appear. These are a bit less what energetic than Pollywas, by 20 nanometers to be exact, but they get the job done.

Yipes!" cried the abused electrons as they receive their new burst of borrowed sunshine.

36 Instructor’s Guide to Laboratory No. 7

36

Through the maze of additional molecules they travel until they are picked up by a rovingmarauder, NADP+. Capturing the two unsuspecting electrons and two H+ protons thathappen to be floating along, NADP+ changes his name to NADPH+H+.

Rushing away from the scene of the crime, the marauder looses one of his H+ protonsand gets lost in the Stroma, the Land of Darkness. It is here where the work really begins.

The rising tide tickles my feet with a lapping wave. I awaken with a face full of wet sand."Argghh"As I slowly make my way back to the fire, I find only smoldering embers and a quiet

beach. I reached down and grab an apple from the supplies I had brought earlier. An applemade of the same building blocks as the wood that had burned several hours earlier.

"Crunch." The apple is fresh and filled with natural sugar. How did it get there?

Once the marauding NADPH reaches the Land of Darkness within the chloroplast, hedelivers his booty of H+ protons to the sugar cycle. These protons are added to a mix ofcarbon dioxide and other carbon molecules to produce PGAL. This in turn is sent to theglucose factory to produce glucose, the basic building block of all plant life.

As I finish my apple and find my way to the car, my mind drifts through the confines ofmy car's engine. Might the gasoline, made from oil, formed in the ground by partiallydecayed plant material, also be borrowed sunshine? I feel my forehead and ask myself, "do Itoo owe this heat to the power of the sun?"

Polly has long since vanished.

References1. Robert Wallace/Jack King/Gerald Sanders, Biology, the Science of Life, 2nd edition,Scott, Foresman and Company, 1986. This is one of the better descriptions of photosynthesis,although there are some confusing points.


37

Laboratory No. 7

Living on Borrowed Sunshine

PurposeThe purpose of this particular lesson to gain a better understanding of the complex pathwaysof photosynthesis through creative writing.

Required Equipment and SuppliesPaper, pen, and a creative spirit.

ActivityYour mission is to simplify the complexities of photosynthesis. Select one of the two

major parts of the process, either the light dependent or light independent reaction, and writea creative story or plan a skit with a group of five to seven other students. Give names to yourcharacters such as Mr. Z for the z particle that breaks up the water molecule or distinctpersonalities to major players like the photons.

Review Questions1. What is the purpose of photosynthesis?

2. What role does water play in the light dependent reaction?

3. Are photons “used,” “transformed,” or “burned” during photosynthesis? Explain.

4. What does the energy obtained from photons actually do?

5. Can the light independent process, often called the dark cycle, occur during both the dayand night? Explain.

Activity

Name: Class: Date:

38 Living on Borrowed Sunshine Laboratory No. 7

38

The Electromagnetic SpectrumInfrared Radiation

39

Instructor’s Guide to Labs No. 8 & 9:

Infrared Radiation and theInverse-Square Rule

and

Detecting Infrared RadiationUsing a Prism

GoalThe goal of these two lab exercises is to introduce the infrared region of the electromagneticspectrum to students through three different explorations.

ObjectivesAfter the explorations the students will be able to:• Understand which regions of the electromagnetic spectrum penetrate the earth's

atmosphere.• Identify similarities in magnetic waves.• Explain three ways infrared radiation can be detected.• Explain the relationship between the number of rotations and the distance from

the source when using the radiometer.• Predict the results of this relationship using a mathematical model (inverse-

square law).• Determine the temperature beyond the red region.• Understand the transmissive and reflective properties of infrared radiation by

using a photodetector setup.


There are three forms of radiation: electromagnetic (EM), mechanical, and particle. These

three experiments will be focusing on electromagnetic radiation. Electromagnetic radiation is

sometimes referred to as light or radiant energy. Electromagnetic radiation travels outward

from its source as waves (pulses) or photons (packets) of energy. The speed of a photon or

EM wave in a vacuum is the same no matter how much energy it carries. This speed is

referred to as the speed of light which is equal to 299,792,456 m/sec and represented by the

letter c.

This section deals with the infrared region of the spectrum. Vibrations and rotations of

atoms and molecules and the motions of their electrons produce this region of the spectrum.

The nature of infrared (IR) has been given in the student exploration. It is important that the

students understand the properties of IR. It can be transmitted, absorbed or reflected. These

properties are also characteristics of the other regions of the EM spectrum.

Experiment

40 Instructor’s Guide to Labs No. 8 & 9

40

Helpful Hints

• Resources:

Everything needed to conduct this experiment is included in the Resource Box.

Descriptions below of materials other than what is in the Box is for informational

use.

• Student Handouts:

This unit is comprised of two separate laboratory exercises, and includes separate

student handouts for each. Refer to each handout for details about materials and

procedures. Note that the discussion and review questions are duplicated in each lab

so that they may be used as individual units if time dictates.

• Lab No. 1:

The first lab uses a hotplate. The one I use is approximately 11 x 11 cm. (corning). I

use the high setting. You may want to try the experiment using the hotplates available

at your site. You may have to vary the temperature setting, etc. I also use any metal

baking pan available to raise the hotplate up to the radiometer’s level.

The mathematical relationship the students should be able to see is that one

physical quantity (number of rotations) varies as the inverse square of the distance

from its source. This is referred to as the inverse-square law. Depending on the data

collection skills used by the students, this law can be seen in the individual

experiments. It can readily be seen when the students data are pooled together and

averaged for each distance-time interval.

• Lab No. 2:

I strongly suggest that you test this setup in advance. It works well with correct

size box and light source. The students will be able to record a noticeable difference

in the temperature increase of the IR region just beyond the visible red region.

This experiment also can be done using the large diffraction grating to disperse

the spectra.


41

Laboratory No. 8:Infrared Radiation and the

Inverse-Square Law

PurposeThe purpose of these labs are to investigate the infrared (IR) region of the electromagneticspectrum using devices that detect IR sources.

Required Equipment and SuppliesHot plate, radiometer, pan, or other item to raise hot plate, metric ruler, graph paper, andstopwatch.

Discussion

One form of radiation is electromagnetic or radiant energy. Sunlight is a familiar form ofelectromagnetic radiation. Only the visible radiation and parts of the infrared and radioregions penetrate the atmosphere completely. Due to absorption by atmospheric nitrogen andoxygen none of the short-wavelength, high-energy gamma rays, x–rays, and short-wavelength (up to 210 nm) ultraviolet radiation make it through. Stratospheric ozone (O3)eliminates another section of the UV band, between 210 and 310 nm.

The various parts of the electromagnetic spectrum produce very different effects whenthey interact with matter but they all travel at the same speed in a vacuum 299,792,456 m/sec(speed of light). The wavelength range between about 750 nm to 1,000,000 nm (or 1 mm) iscalled the infrared region. William Herschel discovered this part of the electromagneticspectrum when he placed a thermometer just outside the red end of the color spectrum. Itregistered a large temperature increase. Hence, infrared radiation may be detected as heat.The heat you feel from a fireplace, campfire, sunlight, or the ground are all sources ofinfrared radiation.

Many living things emit infrared radiation. Rattlesnakes (pit-vipers) have a special pitorgan that is sensitive to infrared radiation and allows them to see minute temperaturevariations in their environment. Detecting small temperature variations allows the snake todetect its prey even in the “darkest of burrows.” The radiometer is a device that was inventedby Sir William Crookes in 1875 to demonstrate the mechanical effect of light radiation. Laterit was used to detect and measure the intensity of infrared radiation. The radiometer is apartially-evacuated tube which contains a structure with four vanes. Each vane has a dark(black) and a light (silver) side. The dark side absorbs much of the infrared radiation and thelight side reflects more than it absorbs. The free molecules present in the tube gain energyand react more with the dark side and push the dark side away from the radiation source. Thespeed of rotation indicates the amount of radiation. Radiometers have been replaced by solid-state electronic devices that measure radiant energy more accurately.

Experiment

Name: Class: Date:

42 Infrared Radiation and the Inverse-Square Law Laboratory No. 8

42

Review Questions1. Which regions of the electromagnetic spectrum penetrate the earth’s atmosphere completely?2. Which waves do not penetrate the earths atmosphere?3. What do all electromagnetic waves have in common?4. What are three ways you can detect infrared radiation?5. How can a pit viper tell that a mouse is hiding in a very dark place?6. Can you see infrared waves? Explain.

Procedure1. Place hot plate on its side and plug it in. The hot plate must be at approximately the same level as

the vanes of the radiometer (place on block or pan).2. Turn on the hot plate and give it two minutes to warm up.3. After the hot plate warms up you will be setting the radiometer in front of it about 24 cm away.

You are to observe the number of rotations the vanes on the radiometer make in a two-minuteperiod.

4. Move the radiometer to 22 cm observing the number of rotations for another 2 minutes. Continueto move the radiometer in at 2 cm intervals and record for two minutes at each interval. Recordthe number of rotations on your data table. Repeat this step until you reach 6 cm.

6. Record data in table below.

TRIALDISTANCE

FROM SOURCE TIMENUMBER OFROTATIONS

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Analysis of Experiment1. Graph the number of rotations and the distance.2. Did distance affect the radiometer's rotation?3. What mathematical model can you use to predict the results?4. Explain the journey of the infrared radiation from the moment it leaves the hot plate to

the point at which the radiometer begins to rotate.


43

Laboratory No. 9:Detecting Infrared Radiation

Using a Prism

PurposeThe purpose of these labs are to investigate the infrared (IR) region of the electromagneticspectrum using devices that detect IR sources.

Required Equipment and SuppliesBox (24 in. long and ~8–12 in. high), quartz bulb with socket, prism, thermometer, 8 x 12 in.backboard (to display spectra), and black sheet of paper. Optional: diffraction grating

Discussion

One form of radiation is electromagnetic or radiant energy. Sunlight is a familiar form ofelectromagnetic radiation. Only the visible radiation and parts of the infrared and radioregions penetrate the atmosphere completely. Due to absorption by atmospheric nitrogen andoxygen none of the short-wavelength, high-energy gamma rays, x–rays, and short-wave-length (up to 210 nm) ultraviolet radiation make it through. Stratospheric ozone (O3) elim–inates another section of the UV band, between 210 and 310 nm.

The various parts of the electromagnetic spectrum produce very different effects whenthey interact with matter but they all travel at the same speed in a vacuum 299,792,456 m/sec(speed of light). The wavelength range between about 750 to 1,000,000 nm (or 1 mm) iscalled the infrared region. William Herschel discovered this part of the electromagneticspectrum when he placed a thermometer just outside the red end of the color spectrum. Itregistered a large temperature increase. Hence, infrared radiation may be detected as heat.The heat you feel from a fireplace, campfire, sunlight, or the ground are all sources ofinfrared radiation.

Many living things emit infrared radiation. Rattlesnakes (pit-vipers) have a special pitorgan that is sensitive to infrared radiation and allows them to see minute temperaturevariations in their environment. Detecting small temperature variations allows the snake todetect its prey even in the "darkest of burrows." The radiometer is a device that was inventedby Sir William Crookes in 1875 to demonstrate the mechanical effect of light radiation. Itwas later used to detect and measure the intensity of infrared radiation. The radiometer is apartially evacuated tube which contains a structure with four vanes. Each vane has a dark(black) and a light (silver) side. The dark side absorbs much of the infrared radiation and thelight side reflects more than it absorbs. The free molecules present in the tube gain energyand react more with the dark side and push the dark side away from the radiation source. Thespeed of rotation indicates the amount of radiation. Radiometers have been replaced by solid-state electronic devices that measure radiant energy more accurately.

Experiment

Name: Class: Date:

44 Detecting Infrared Radiation Using a Prism Laboratory No. 9

44

Review Questions1. Which regions of the electromagnetic spectrum penetrate the earth’s atmosphere

completely?2. Which waves do not penetrate the earths atmosphere?3. What do all electromagnetic waves have in common?4. What are three ways you can detect infrared radiation?5. How can a pit viper tell that a mouse is hiding in a very dark place?6. Can you see infrared waves? Explain.

Procedure1. Place quartz bulb in box at one end.2. Cut a slit approximately 5 mm x 15 mm in the opposite end of the box.3. Place the black piece of paper 3–5 cm inches from bulb (this paper acts as a partition

between the bulb and the other end of the box and helps to focus the light rays). Punch ahole at about the same level as the slit.

4. Place a parallel white piece of paper approximately 20 cm away from slit. Place prism infront of the slit. Turn on light and move prism until a visible spectrum comes into focus.

5. Record the temperature of the following areas:(a) Room (away from spectrum area)(b) Visible area of spectrum (red, orange, yellow, green, blue, and violet)(c) The dark region just past the red.

Analysis of Experiment1. Is the region beyond the red hotter or cooler than the visible area?2. Why would the region beyond the red be hotter than other areas of the visible region?3. What generalization(s) can you make about the region beyond the red and what evidence

can you state to support your generalization(s)?


45

Instructor’s Guide to Lab Nos. 10a–b:

Investigation of IR LightUsing an IR Transmitter and Receiver

and

Investigation of IR LightUsing a Close-Circuit TV Camera

GoalThe goal of this lab to investigate the transmission and propagation of infrared (IR) light and tointroduce the concept of lightwave communications using an IR transmitter and receiver.


• Have a feeling for the existence and use of “invisible” light.• Understand how a typical TV remote control works.• Understand the concept of light-wave communications.• Understand the phenomena of light-wave reflection, refraction, and absorption.• Use a prism and diffraction grating to disperse IR light in the same manner as visible light.


Infrared waves are electromagnetic waves with frequencies lower than visible light. The lowestfrequencies of visible light are red, so we call the lower frequencies infrared, meaning beyond red.This type of electromagnetic radiation is widely used for local communications, in which the senderand receiver are very close together, such as a VCR remote control (transmitter) with the VCRreceiver. Infrared light is sent by the VCR remote using a special electronic device called an emittingdiode, which emits light when an electric current is passed through it. Behind a window in the VCR isa matched diode which passes current when it absorbs infrared light. The transmitter uses infraredlight to carry information by modulating the signal, usually using a series of short on/off pulsessimilar to Morse code, which are received and decoded by the receiver and circuitry in the VCR.

This lab is separated into two versions, A and B. Version A lab calls for a matched IRtransmitter/Receiver pair. This setup can be built for less than $20 using parts available fromRadioShack in the circuit shown below.1 It is a simple amplitude-modulated light wave communi-cations system. Alternatively, just the receiving end can be built and used to “listen” to the signalsproduced by light sources such as an IR remote control. Allowing for interchangeable photodiodes (toswitch between visible and infrared) in the circuit makes it even more versatile. Students can use it tohear what their TV remote control is saying (a series of tones.) An incandescent lamp will produce ahum, a fluorescent lamp a buzz, and an electronic camera flash will produce a large pop. A flashlightbeam can be swept slowly across the light listener’s detector to produce a soft swishing sound, whilea fast sweep will produce pops. Tap the flashlight with a pencil and a ringing sound will be heard asthe filament vibrates. Interesting!

Version B of this lab calls for an ordinary CCD or viticon-based TV camera. The semiconductordetector chips in some of these are very sensitive to IR. This setup is used in place of the IRTransmitter/Receiver in the B version of this experiment. We have obtained a number of surplussecurity cameras, and included one in each Resource Box. Point any type of IR-based remote controlunit at the camera and it appears as a bright source on the monitor. Also use the included “IRFlashlight” (an infrared LED wired into an ordinary flashlight in place of the normal bulb) with the

Experiment

46 Instructor’s Guide to Labs No.s 10a–b

46

included prism and/or diffraction grating to show diffraction. Use developed color film as an IRbandpass filter to filter out excess visible. The lights can be turned out and the IR flashlight can beused to illuminate students. Watch out, you can be observed in the darkest of nights! A lot can bedone with this set up. Be imaginative, and let us know what you come up with!

Helpful Hints

• A simpler circuit2 is shown in Figure 2. The source can be modulated by turning on/off, or atypical TV remote control can be used.

• This lab can be done using toy ray guns3. The Photon© ray gun, made by Entertech, or theBravestarr Evil Laser-Fire Backpack© by Mattel can be used.

References

1. Forest M. Mimms, Engineers Mini-Notebook: Communications Projects, pg. 28-29 (1994).Available at RadioShack stores.

2. Forest M. Mimms, Engineers Mini-Notebook: Communications Projects, pg. 24-25, (1994).Available at RadioShack stores.

3. R. S. Halada, "Demonstrations of Infrared Ray Optics Using Ray Guns," Physics Teacher 29, 370(1991).


47

Laboratory No. 10a:Investigation of the Properties of LightUsing an IR Transmitter and Receiver

PurposeThe purpose of this lab is to investigate the transmission and propagation of light using aninfrared transmitter and receiver.

Required Equipment and SuppliesIR transmitter, IR receiver, focusing tube with lens, glass plate, clear, tinted and opaqueplastic plates, IR prism, pocket mirror, white card, and black card.

Discussion

Infrared waves are electromagnetic waves with frequencies lower than visible light. Thelowest frequencies of visible light are red, so we call the lower frequencies infrared, meaningbeyond red. This type of electromagnetic radiation is widely used for local communications,in which the sender and receiver are very close together, such as a VCR remote control(transmitter) with the VCR receiver. Infrared light is sent by the VCR remote using a specialelectronic device called an emitting diode, which emits light when an electric current ispassed through it. Behind a window in the VCR is a matched diode which passes currentwhen it absorbs infrared light. The transmitter uses infrared light to carry information bymodulating the signal, usually using a series of short on/off pulses similar to Morse code,which are received and decoded by the receiver and circuitry in the VCR.

Review Questions1. Can you see infrared waves? Explain.

2. What is a common use for Infrared waves? Can you think of another?

Procedure A: The Transmission and Reception of IR Light1. Assemble the transmitter and receiver by hooking up the speaker to the receiver and the

radio to the transmitter.

2. Turn on the transmitter and receiver and place in front of one another so that the diodesare in close proximity and facing each other.

3. Turn on the radio and align the diodes until you can hear the radio playing. Tune theradio to find a strong signal from a nearby radio station.

4. Move the receiver away from the transmitter slowly and carefully. Try to keep the diodesaligned so that the radio continues to play through the speaker.

5. Determine the maximum range of the transmitter/receiver pair by slowly moving thereceiver away until the signal breaks-up into random noise.

6. Now place the receiver at a distance from the transmitter of about one-half of themaximum range. Carefully align the receiver to pick up the best signal.

7. Now place the glass plate between the receiver and the transmitter and note what happensto the signal.

Experiment

Name: Class: Date:

48 IR Transmitter/ Receiver Laboratory No. 10a

48

8. Repeat (7) with the clear, tinted, and then opaque plastic sheets, noting what happens tothe signal in each case.

Procedure B: The Reflective, Refractive, and Transmissive Properties of Light.1. Bring the receiver and transmitter together so that the diodes are directly facing each

other, with the transmitter in front of you and facing away. Make sure a clear signal isbeing picked up by the receiver.

2. Now place the receiver at a 45° angle to the transmitter.

3. Place the prism directly in front of the transmitter diode and rotate it until the radio isheard through the speaker, indicating a signal is being received. Note the position andorientation of the prism.

4. Replace the prism with the pocket mirror, orienting it until a signal is picked up. Note theposition and orientation of the mirror.

5. Repeat (4) using the white card and then the black card. Note whether or not the signal isbeing received, and the position and orientation of the cards.


1. Explain how the music is traveling from the transmitter to the receiver.

2. Explain the effect the prism had on the signal while in front of the transmitter. Whatproperties of light are being exhibited: refraction, reflection, absorption, and/ortransmission?

3. Explain the effect the mirror had on the signal while in front of the transmitter. Whatproperties of light are being exhibited?

4. Explain the effect the white card had on the signal while in front of the transmitter. Whatproperties of light are being exhibited?

5. Explain the effect the black card had on the signal while in front of the transmitter. Whatproperties of light are being exhibited? Where did the energy of the signal go?


49

Laboratory No. 10b:Investigation of IR Light

Using a Close-Circuit TV CameraPurposeThe purpose of this lab is to investigate the transmission and propagation of light using aninfrared transmitter and receiver.

Required Equipment and SuppliesClosed-circuit Black and White TV camera and monitor, Source of IR light (TV remotecontrol transmitter and/or IR light emitting diodes (LEDs) and power supply), pieces ofdeveloped color film.

Discussion

Infrared waves are electromagnetic waves with frequencies lower than visible light. Thelowest frequencies of visible light are red, so we call the lower frequencies infrared, meaningbeyond red. This type of electromagnetic radiation is widely used for local communications,in which the sender and receiver are very close together, such as a VCR remote control(transmitter) with the VCR receiver. Infrared light is sent by the VCR remote using a specialelectronic device called an emitting diode, which emits light when an electric current ispassed through it. Behind a window in the VCR is a matched diode which passes currentwhen it absorbs infrared light. The transmitter uses infrared light to carry information bymodulating the signal, usually using a series of short on/off pulses similar to Morse code,which are received and decoded by the receiver and circuitry in the VCR.

The rods and cones in our eyes don’t respond to IR light, so we say that it is invisible.However, the semiconductor detector chips in some types of black and white TV cameras aresensitive to IR and so can “see” it. The black and white picture on the TV tube the camerashows us, then, will show IR light. Hence, we can use the camera to image IR light from aremote control or other device. In fact, a high-power IR lamp can be used to illuminate alarge area at a distance, without us humans even knowing it. The TV camera, though, and theperson viewing through it, can see us. Watch out, you can be observed in the darkest ofnights by invisible IR light beams!

Review Questions1. Can you see infrared waves? Explain.

2. What is a common use for Infrared waves? Can you think of another?

3. Why does IR light show up on the monitor of a black and white TV camera?

Procedure A: The Transmission and Reception of IR Light1. Connect the TV camera and monitor together, and focus the camera on an object several

meters away or so.

2. Standing several meters away, aim and “shoot” the remote control at the TV camera andobserve what happens on the monitor.

Experiment

Name: Class: Date:

50 IR Light Viewed Using a TV Camera Laboratory No. 10b

50

3. Have a partner hold some developed color film in front of the camera while you shoot theremote. Observe changes in the background light and that from the remote.

4. Have a partner hold a piece of developed color film in front of the camera while youshoot the remote. Observe changes in the background light and that from the remote.

5. Turn out the lights and watch the TV monitor while you shoot the remote. Observechanges in the background light and that from the remote.

Procedure B: The Reflective, Refractive, and Transmissive Properties of Light.1. Bring the camera and IR light source (either an IR LED or a TV remote control) together

so that the diode is directly shining into the camera. Make sure a clear image is beingpicked up by the camera and displayed on the monitor.

2. Now place the transmitter at a 45° angle to the camera

3. Place the prism directly in front of the transmitter diode and rotate it until the image ofthe diode is seen on monitor. Note the position and orientation of the prism.

4. Replace the prism with the pocket mirror, orienting it until a signal is picked up. Note theposition and orientation of the mirror.

5. Repeat (4) using the white card and then the black card. Note whether or not the signal isbeing received, and the position and orientation of the cards.

6. Align the transmitter diode and camera so a clear image of the diode is seen. Place adiffraction grating in front of the camera lens. Note any changes in the image – this is thefirst-order image of the diode. Now move the transmitter along a line perpendicular to thecamera, keeping the diode pointed at the camera lens/grating. Stop when you see the first-order image.

7. Your teacher may want you to measure the wavelength of the IR light emitted by thediode. This can be done by measuring the distances involved. Can you figure out how?Recall earlier labs in which you measured the wavelengths of various colors of visiblelight using a diffraction grating.


1. How is the image of the IR LED being generated on the TV monitor?

2. Explain the effect the prism had on the signal while in front of the IR diode (transmitter).What properties of light are being exhibited: refraction, reflection, absorption, and/ortransmission?

3. Explain the effect the mirror had on the signal while in front of the IR transmitter. Whatproperties of light are being exhibited?

4. Explain the effect the white card had on the signal while in front of the IR transmitter.What properties of light are being exhibited?

5. Explain the effect the black card had on the signal while in front of the IR transmitter.What properties of light are being exhibited? Where did the energy of the signal go?

6. Optional: Determine the wavelength of the IR light from the source you used.

The Electromagnetic SpectrumUltra-Violet Light

51


FluorescenceGoalTo investigate the phenomena known as fluorescence and phosphorescence.

ObjectivesAfter doing this lab or observing this demonstration students will be able to:

• Understand what causes fluorescence.

• Understand how fluorescence can be used.

• Understand what causes phosphorescence .

• How to detect ultraviolet light using fluorescent material

Background InformationWhen an atom is excited from one energy state to a

higher stage by the absorption of a photon, it may returnto the lower level in a series of two (or more) jumps ifthere is an energy level in between. The photons emittedwill consequently have lower energy and frequencythan the absorbed photon. This phenomenon is calledfluorescence; common fluorescent rocks and paints canemit visible light after absorbing UV light.

Fluoresence is responsible for the appearance of objects under the so-called “black light,” whichis a source of ultraviolet radiation. Photons in the ultraviolet region, invisible to the human eye, havehigher energies than those in the visible region, and hence if an ultraviolet photon is absorbed by anatom, the outer electron (which is responsible for the visible transitions) can be excited to high levels.These electrons make transitions back to their ground state, accompanied by the emission of photonsin the visible region.

Objects seen in ultraviolet light often show colors in the blue or violet end of the spectrum whichare not present when the objects are viewed in sunlight; common fluorescent rocks and paints canemit visible light after absorbing invisible UV light. We can understand this effect by considering thecomposition of sunlight and the optical excited states of a typical atom. The intensity of sunlight isconcentrated in the center of the visible spectrum in the yellow region; very little intensity is presentin the red or blue ends of the visible spectrum. The “yellow” photons have enough energy to excite anatom up to its lower levels, but not enough to reach the higher levels. However, the higher-energy UVphotons do have sufficient energy to reach the higher level, so the light emitted by the atom has astronger blue component when that atom is excited by ultraviolet light than when excited by sunlight.

The wavelength for which fluorescence will occur depends on the energy levels of the particularatoms. Because the frequencies are different for different substances, and because many substancesfluoresce readily, fluorescence is a powerful tool for identification of compounds. It is also used forassaying – determining how much of a substance is present–and for following substances along anatural pathway as in plants and animals. For detection of a given compound, the stimulation lightmust be monochromatic, and solvents or other materials present must not fluoresce in the same regionof the spectrum. Often the observation of fluorescent light being emitted is sufficient; in other cases,spectrometers are used to measure the wavelengths and intensities of the light.

Fluorescent light bulbs work in a two-step process. The applied voltage accelerates electrons thatstrike atoms of the gas in the tube and cause them to be excited. When the excited atoms jump downto their normal levels, they emit UV photons which strike a fluorescent coating (called a phosphor) on

Activity

Onephoton

absorbed

Twophotonsemitted

Eo

E1

E2


52

the inside of the tube. The light we see is a result of this material fluorescing in response to the UVlight striking it.

Materials such as those used for luminous watch dials are said to be phosphorescent. In a phos-phorescent substance, atoms can be excited by absorption of a photon to an energy level said to bemetastable. When an atom is raised to a normal excited state, it drops back down within about10-8 sec. Metastable states can be much longer – even a few seconds or longer. In a collection of suchatoms, many of the atoms will descend to the lower state fairly soon, but many will remain in theexcited state for over an hour. Hence, light will be emitted even after long periods. When you put awatch dial close to a bright lamp, it excites many atoms to metastable states, and you can see the glowa long time after.

Helpful Hints• The optional UV viewer described in the lab requires a UV bandpass filter. We have built one of

these for each Resource Box using a PVC pipe fitting. Use the “invisible ink” to make thedetector screen.

ReferencesTom Donohue and Howard Wallace, "Ultraviolet Viewer," Physics Teacher 31, 41 (1993).


53

Laboratory No. 11:

FluorescencePurpose

To investigate the properties of UV light using the process of fluorescence.


Long-wave ultraviolet lamp (black light), short-wave UV lamp, fluorescent mineral set, invisible ink,fluorescent crayons, UV detector cards, (optional: UV bandpass filter, tube to make UV viewer.)

Discussion

We have learned that atoms can become excited by absorbing light, and also de-excited byemitting light. The atoms of each element have a unique set of wavelengths for the absorption andemission of light, depending on the different possible energy states the atom possess. Many atoms canabsorb invisible light such as ultraviolet and then spontaneously emit less energetic visible light. Thisway of giving off light is called fluorescence, and is responsible for the appearance of objects underso-called “black light,” which is a source of ultraviolet radiation. Objects seen in ultraviolet lightoften show colors in the blue or violet end of the spectrum that are not present when the objects areviewed in sunlight. Common fluorescent rocks and paints can emit visible light after absorbinginvisible UV light.

We can use fluorescent materials to test for the presence of invisible ultraviolet radiation byputting an object under a special light filter that blocks out all light except UV. If the materialfluoresces, then there is UV present. If it is dark, then there is no UV.

A familiar example of fluorescence is the common fluorescent lamp you might be sitting underright now. In a fluorescent lamp, oscillating electrons excite atoms of mercury gas, which then giveoff photons of intense and invisible ultraviolet light. The inside surface of the lamp is covered with apowdery fluorescent material called a phosphor, and this material first absorbs the ultraviolet photonsand then emits photons of visible light. The excited atoms in the phosphor take several steps, ortransitions, to return to their original energy (ground) state. Each step results in the emission of lessenergetic photons that have frequencies in the range of visible light, which combine to produce white-looking light. Different phosphors can be used to produce different colors of light.

Because the fluorescence frequencies are different for different substances, fluorescence is apowerful tool for identification of rocks, minerals, and other compounds. It is also used for assaying –determining how much of a substance is present – and for following substances along a naturalpathway as in plants and animals.

Some materials have excited states which are metastable, where the transition to a lower energystate is not spontaneous and takes more time. Materials that exhibit this peculiar property are said tohave phosphorescence. The element phosphorus, which is used in luminous clock dials and in otherobjects that are made to glow in the dark, is a good example. Atoms or molecules in these materialsare excited by incident visible light. Rather than de-exciting immediately, as fluorescent materials do,many of the atoms remain in a state of excitement, sometimes for as long as several hours – althoughmost undergo de-excitation rather quickly. If the source of excitation is removed – for example if thelights are put out – an afterglow occurs while millions of atoms spontaneously undergo gradual de-excitation.

Activity

Name: Class: Date:

54Fluorescence Laboratory No. 11

54

A TV screen is slightly phosphorescent, the glow decays rather quickly, but just slowly enough sothat successive scans of the picture blend into one another. The afterglow of some phosphorescentlight switches in the home may last more than an hour. The same is true for luminous clock dials,excited by visible light. Some older clock dials glow indefinitely in the dark, not because of a longtime delay between excitation and de-excitation, but because they contain radium or some otherradioactive material which continuously supplies energy to keep the excitation process going. Suchdials are no longer common because of the potential harm of the radioactive material to the user.

Examples of phosphorescence are found in living creatures – from bacteria to fireflies and largeanimals like jellyfish. These creatures chemically excite molecules in their bodies that give off light, aprocess we call bioluminescence. The firefly uses a chemical reaction to emit light so it can be seen.However, certain squid emit visible light to become invisible! To fool predators below, these squiduse light as camouflage by carefully regulating their brightness to match the intensity of the sunlightat their depth. Under some circumstances certain fish become luminescent when they swim, butremain dark when still. Schools of these fish hang motionless and are not seen, but when alarmed theystreak away with a sudden burst of light, creating a sort of deep sea fireworks. The mechanism ofbioluminescence is not yet well understood and needs to be carefully investigated, perhaps by futurescientists such as yourselves.

Review Questions1. What kind of light is sometimes called “black light”?2. What causes materials to be fluorescent?3. What is a common example of fluorescence?4. What is the main difference between fluorescence and phosphorescence?5. What is an example of bioluminescence?

Procedure1. Draw a picture using the special fluorescent crayons provided in the kit. Take some time to draw

a decent picture.1. Set up the black light contained in the kit. Also set up the UV lamp, BUT DO NOT TURN IT

ON.3. Using the invisible ink, write something witty on the cardboard screen that will be inserted into

the detector. Place the screen into the detector.4 Try and detect the presence of UV light at different places in the classroom. If you can, take the

detector outside into sunlight and record any changes in the image. Try and detect UV with theUV detector card. Record your observations.

5. Turn on the black light lamp. Use the UV detector to try and detect the presence of ultravioletlight. Try and detect UV with the UV detector card. Record your observations.

6. Place each of the various minerals contained in the kit under the black-light lamp. Record whatyou see. Note which minerals respond, and with what colors.

7. Place your picture under the lamp and observe the changes in the colors.8. Put on your UV-goggles, and turn on the UV lamp. Leave the black light turned on. Use the UV

detector and record any differences you can observe.9. Record any changes you can observe in the minerals and your artwork. Note which minerals

respond, and with what colors.10. Now turn off the black light and observe and record any changes in the fluorescence of the

minerals, your artwork, and the UV detector.11. Turn off the UV lamp. Remove goggles.


55

Instructor’s Guide to Labs No. 12 & 13

UV Light Detectionand

Absorption of UV Light by Oxygen

GoalTo test various types of plastic and glass for UV transmittance, and to investigate theabsorption of UV light by oxygen.

ObjectivesAfter completing these two labs students will be able to:

• Determine which brand of sunglasses block UV light most effectively.• Determine which types of plastic and glass allow UV transmittance.• Discuss the formation and destruction of ozone.• Be able to work with UV light safely.• Determine what happens to the transmittance of UV as the concentration of oxygen

increases.


For certain applications it is necessary to know how well various materials transmit orblock UV light. Sunglasses are a good example. See the discussion section in the studenthandouts for more information.

Cautions• Students working with the UVA and UVB lamps must wear goggles at all times since

UV light can damage the retina.• The resin must be poured and kept in a well-ventilated area during the entire experiment.

Helpful Hints – Gas absorption apparatus• Make sure the hose and stopper fit gas tight.• Make sure the flask is cleaned out well in between uses.• The UV light should be 10 cm above the gas testing device. It can be hooked to a ring-

stand for convenience.

ExtensionsOther gases besides oxygen can be tested with the gas absorption apparatus.

Experiment

56 Instructors Guide to Lab Nos. 12 & 13

56


57

Laboratory No. 12:

UV Light DetectionPurposeTo test various types of plastic and glass for UV transmittance.

Required Equipment and SuppliesSome type of UV light detector, UV filter, UV light, five types of clear plastic, five types ofclear glass, one pair of sunglasses brought by each student, five clear plastic or glass itemsfrom the classroom, ring stand, two clamps, goggles.

Caution: Always wear plastic goggles when working with UV light since UV cannot passthrough plastic.

DiscussionSome types of glass and plastic will stop ultraviolet (UV) light while others are

transparent to it. This lab will explore the degree to which types of glass, plastic, andsunglasses are opaque or transparent to UVlight.

Procedure1. Hang the UV light on the ring stand so

that it is 15 cm above the surface of thetable.

2. Place the clamp to hold the materialsamples 3 cm below the UV lightsource.

3. Place the first type of plastic into theclamp.

4. Place the UV light detector directly underneath the sample to be tested.

5. Put on goggles and then turn on the UVlight source.

6. Record the type of plastic and thetransmittance in a table.

7. Repeat for the four remaining types ofplastic, five types of glass, each type ofsunglasses, the UV filter, and the fiveassorted lab items.

Analysis of Experiment1. Make a bar graph showing the type of material versus the transmittance.2. Which type of plastic was best? Worst?3. Which type of glass was best? Worst?4. Rank order the sunglasses by brand from best to worst for UV protection.5. Can you say how the absorption characteristics of each material are related to the

transmittance values recorded?

Experiment

Name: Class: Date:

UV Light Source

Sample (glass or plastic)

UV Light Detector

Figure 1 The UV light testing apparatus.

58 UV Light Detection Laboratory No. 12

58


59

Laboratory No. 13:

Investigating the Absorptionof UV Light by Oxygen

PurposeTo investigate the absorption of UV light by Oxygen.

Required Equipment and SuppliesGas testing apparatus, Liquid Crystal UV light detector, UV light, rubber tubing, rubberstopper, 250 ml Erlenmeyer flask, manganese dioxide, hydrogen peroxide (3%), massbalance, goggles.


Discussion

The earth is surrounded by a thin layer of ozone gas present in the stratosphere whichabsorbs almost all of the incoming UV light from the sun. Without this shield, life on thisplanet would have evolved very differently.

Ozone (O3) is produced when atmospheric oxygen (O2) is dissociated by UV radiationinto two individual oxygen atoms, and one of these atoms combines with O2, forming O3.

O UV Radiation O O

O O Mediator O Mediator2

2 3

+ → ++ + → +

Ozone itself can then be dissociated by UV light into O2 and O. The amount of ozone in theearth’s atmosphere is about 3 billion tons. Despite this high number, if this amount of ozonewere moved to the earth’s surface, it would form a layer only 3 mm thick. It exists in theatmosphere in a concentration of only about twenty to thirty parts per million.

The destruction of the earth’s ozone layer is a major environmental concern. Manychemicals used by industries and consumers actively destroy ozone. These chemicals includechloro-fluorocarbons used to produce some types of styrofoam, freon used in refrigerationand air-conditioning units, and spray cans that contain fluorocarbons. A large effort is beingundertaken by environmentalists and chemists together to find and use alternatives to thesedestructive products.

Ironically, ozone itself is produced artificially at the earth’s surface by industrialprocesses and the combustion of fossil fuels, and is a serious pollution problem for majorcities like Los Angeles. Ozone is a noxious pollutant which can hurt eyes, plants, and destroyrubber products.

Review Questions1. Where is most of the earth’s ozone concentrated?2. What is the chemical formula of ozone gas?3. Write the chemical equations which describe how ozone is produced, then destroyed.4. How much ozone is there in the atmosphere?5. What is the concentration of ozone in the atmosphere?6. How can ozone also be a pollutant?

Experiment

Name: Class: Date:

60 Investigating the Absoption of UV Light by Oxygen Laboratory No. 13

60

Procedure

1. Set up the gas testing apparatus and ringstand assembly as shown in Figure 1.Place the liquid crystal UV detector directly underneath the gas testing device.Do not leave any air space between thedetector and the gas testing device: thefigure shown is an exploded view forclarity

2. Put the rubber hosing onto the device andthe stopper into the flask

3. Place the UV light 3 cm above the gastesting device.

4. Put on the goggles and turn on the UVlight.

5. Record the transmittance on the UVdetector for the atmosphere, then turn outthe UV light.

6. Remove the stopper from the flask.

7. Measure out 0.1 g of manganese dioxide and place it in the flask.

8. Add 20 ml of hydrogen peroxide and quickly stopper the flask.

9. Wait until the reaction in the flask has stopped.

10. Turn on the UV light, record the transmittance, then turn out the UV light.

11. Clean out the flask and the gas testing device, and then repeat steps 5 through 9 with30 ml and 50 ml of hydrogen peroxide mixed with 0.1 g of manganese dioxide eachtime.

12. Record the UV transmittance after each trial.


1. Make a graph indicating the amount of manganese dioxide used (an indication of theamount of oxygen produced) versus UV transmittance.

2. What happened to the UV transmittance as the amount of oxygen increased? Why?

UV Light Source

Gas Testing Device

Flask

UV Light Detector

Figure 1


61


The Effect of UV Light on YeastGoalThe goal of this lab is to introduce students to the effects of UV light on a test organism (theyeast.)

ObjectivesAfter completing this lab, students will be able to:

• Explain the effect of UV light on yeast growth.• Analyze colony growth on agar plates.• Describe where UV light fits into the overall Electromagnetic Spectrum.• Explain why UV light has a deleterious effect on yeast growth.• Demonstrate sterile technique for growing yeasts.• Calculate dilutions for yeast growth.• Accurately graph yeast growth versus exposure time.

Background InformationFound in student handout.

Helpful Hints• UV light sensitive yeast can be purchased from large scientific supply houses.

• Use a germicidal UV lamp if possible. (This is supplied in the Resource Box.)

• Make sure students swirl or vortex mix the yeast suspensions and dilution tubesthoroughly each time.

• Making the nutrient agar plates:(a) Make 10 nutrient agar plates per group.(b) Mix nutrient agar with deionized water according to directions on the label of the

nutrient agar jar.(c) Sterilize at 20 lbs of pressure for 20 minutes.

• Starting the yeast culture: Each group will need one experimental and one controlsample of yeast.

(a) Make 0.1 molar sucrose and pour 15 ml into two test tubes for each group.(b) On the day before the lab, sterilly add the UV sensitive yeast to the test tubes. Let

them grow overnight.(c) On the day of the lab, sterilly pour the yeast suspension into two sterile petri dishes,

one marked “Experimental” and the other “Control.”

• “Plating” the yeast cells:(a) Students dip glass spreader into alcohol and immediately place it into the Bunsen

burner flame. Let the flame go out.(b) Open the lid of the agar plate slightly and gently rub it across the agar 3-4 times to

cool it down. Be careful, the agar can tear!!

Experiment

62Instructor’s Guide to Lab No. 14

62

(c) Remove 100 µl (0.1 ml) from the yeast tube, carefully lift the cover of the agar platejust a crack, and inject the yeast onto the agar.

(d) Immediately turn the spreading dish wheel and move the glass spreader back andforth 5–6 times along it to spread the yeast evenly onto the nutrient agar.

(e) Close the plate cover.

• Store the completed plates up side down at 37° C for 1–2 days in an incubator.

• Make sure students do not open the petri dishes when counting the colonies.

• Analyzing the results: Theoretically, students will see a dose response to increasing timeexposure to UV light. As the length of UV exposure time increases, fewer and feweryeast cells will be observed.

Extensions

• Mix so called antioxidant compounds in with the yeast to see if survivability is increasedover a control.

• Place UV filters (even sunglasses) above the yeast to test for the protective effects.


63

Laboratory No. 14:

The Effect of UV Light on YeastPurposeTo determine the effects of UV light on yeast growth.

Required Equipment and SuppliesTwo yeast cultures in petri dishes, UV lamp, petri dish, incubator; Per group: goggles, 10nutrient agar petri dishes, glass spreader, plating wheel, Bunsen burner, striker, pipetter (orplastic pipette), sterile cover dish. Optional: vortex mixer, plating wheel.

Caution: Always wear plastic goggles when working with UV light since UV cannot passthrough plastic. Sterile technique must be used throughout the lab to ensure proper results.

DiscussionUltraviolet (UV) light has a wavelength range of 5 to 400 nanometers (1 nm = 10-9 m)

placing it between visible light and x–rays on the electromagnetic spectrum (Figure 1). It hasa shorter wavelength and therefore more energy than visible light.

UV at a wavelength between 29 and 31 nm causes melanin production in skin cells ofhuman beings, which causes fairskinned persons to tan. This same wavelength is necessaryfor vitamin D production in human beings. Since UV is a type of ionizing radiation it candamage cells. Overexposure to UV in sunlight can cause sunburns which could lead to skincancer. Only about 1% of the UV from the sun is able to penetrate the earth's atmosphere.Directly looking at UV light can damage the retina of the eye.

Certain insects have eyes which see in UV light. Since some flowers show verydistinctive and different coloration in UV light compared to visible light, this allows insectssuch as bees to distinguish easily among flowers which look very similar in visible light.

Some wavelengths of UV light kill cells and are used to sterilize medical instruments.

Radio Microwave Infrared Visible Ultra-violet X-Ray Gamma Ray

<.01 nm.1 nm1 nm10 nm100 nm1 mm10 mm.1 mm1 mm>1 mm

Red Orange Yellow Green Blue Violet

Shorter Wavelengths = Higher EnergyLonger Wavelengths = Lower Energy

Figure 1

Review Questions1. Where is UV light found in the Electromagnetic Spectrum?2. What is its wavelength range?3. Is UV more or less energetic than visible light? Why?4. Which wavelength causes suntans and sunburns?5. What can these lead to?6. How much of the UV from the sun penetrates the Earth's atmosphere?7. Which kind of animal can ‘see’ in UV light?8. Why can UV light be used to sterilize medical instruments?

Experiment

Name: Class: Date:

64 The Effect of UV Light on Yeast Laboratory No. 14

64

Control Experimement

Figure 2

Procedure– Day 11. Raise the sterile cover and remove 0.1 ml of yeast from the experimental culture and

0.1 ml from the control culture in separate pipetters.2. Plate six nutrient agar petri dishes with the experimental culture and six nutrient agar

petri dishes with the control culture. Label each experimental petri dish as E1 through E4,and the control dish as “Control.”

3. Setup the UV light at a distance of 8 cm above where the petri dish will be placed (seeFigure 2).

4. Place the control dish in the compartment seperated from the UV lamp.5. Tun on the UV lamp.

6. Place the first experimental petri dish (E1) under the UV lamp for 15 seconds and thenquickly remove. Be sure to time the exposure accurately.

7. Repeat step 5 using dishes E2 through E4 for 30, 45, and 60 seconds, respectively.8. Cover the petri dishes and set aside in a place where they won’t be disturbed for two

days.

Day 39. After waiting two days, count the number of yeast colonies on each. Note: do not open

the petri dishes.10. Make a graph of Exposure Time versus Yeast Colonies. Plot the control and experimental

numbers on the same graph.

Analysis of Experiment1. What is the fraction of yeast in the final nutrient agar petri dish?2. What was the reason for the control?3. What was the effect of UV light on the yeast? Why?


65


Effect of UV Light on DNA

GoalTo determine the effects of UV light on DNA.


• Discuss the three forms of DNA which result from UV damage.• Make, load, and run a gel with DNA.• Calculate the quantitative degree of DNA damage which results from exposure to UV

light.


Being able to accurately quantify DNA strand breaks in an isolated system is a fairly newtechnique. The quantification of these breaks can be achieved by using plasmid DNA,electrophoresis, and measuring the square area of the bands produced.

When a DNA system is exposed to ionizing radiation, there are two potentially damagingforces, one direct and the other indirect. The direct source of damage is due to physicaldeposition of energy that disrupts atomic structure, actually ionizing the DNA. The indirectmeans of radiation damage is due to free radical formation. The indirect means of radiationdamage is due to free radical formation. It is this indirect means of damage (i.e., radicalformation) that will be measured in this system.

Electrons located in shells surrounding the nuclei can absorb radiation and move to shellsof higher energy levels. If the energy absorbed is great enough, the electron will escape itsatom or molecule, resulting in a free electron and an atom or molecule which is missing anelectron. These electron-deficient species, also called free radicals, are highly unstable andwill rapidly react with the surrounding environment.

One of the most important radicals is the hydroxyl radical (OH-) not only because it isone of the most reactive species, but also due to its relative abundance. The hydroxyl radicalis generated with radiation by the hydrolysis of water. The OH- is a common radical linkedwith genetic mutation, aging, and DNA strand breakage.

When a hydroxyl radical reacts with DNA, a strand break can occur. This happensbecause a hydrogen atom is ripped off of the deoxyribose sugar of the DNA, therebyquenching the OH- into H20. However, the resulting free electron, through an unknownmechanism, will break the strand. If only one strand break occurs on the supercoiled DNA,an open circular form will be assumed. Similarly, if two strand breaks occur in close enoughproximity, the DNA will assume a linear form.


UV light can damage the retina.

Experiment

66 Instructors Guide to Labs No. 15

66

Helpful Hints• Agarose Gels: (1) Make 1% agarose gels by mixing 1 part agarose to 99 parts 1X-TBE

solution. (2) Make enough to cast the appropriate number of gels for your class.

• TBE running buffer: (1) can be purchased commercially in 50X or 20X stock solutions.(2) Be sure to dilute to appropriate strength before using.

• Plasmid DNA: pUC plasmid can be purchased from commercial scientific supplyhouses. Make sure it is as near to 100% supercoiled as possible!! This will greatlyenhance results!!!

• Loading Dye: Can be purchased from scientific supply houses.• Make sure the DNA is kept on ice at all times. Ice under the UV lamp will melt and will

need constant re-supply.

Analysis of Results• The gels can be stained with ethidium bromide and visualized with UV light or

methylene blue and visualized with white light.• The band migrating the furthest will be supercoiled followed by a middle band of open

circles, and if enough UV is given, a third band of linear.

ExtensionsSubstances known to be free radical scavengers (i.e., vitamins) can be tested in this

system. A free radical scavenger is a molecular species which reacts with free radicals totransform them into either a non-radical species, or a more stable radical. The scavengersusually do this by donating an electron to the free radical, thus stabilizing it.


67

Laboratory No. 15:

The Effect of UV Light on DNAPurposeTo determine the effects of UV light on DNA.


TBE (Tris-borate-EDTA) running buffer, gel box, agarose, power supply, plasmid DNA(pUC18 or BR322), pipette, loading dye (BTB and xylene cyanol), stain (methylene blue,ethidium bromide, or a proprietary DNA stain), UV light, 1.5 ml microfuge tubes, ruler,poloroid camera, goggles, light box.Caution: Always wear plastic goggles when working with UV light since UV cannot passthrough plastic. Sterile technique must be used throughout the lab to ensure proper results.

Supercoiled DNA Single Strand Break- Open Circle Double Strand Break - Linear/Twisted

Discussion

Ultraviolet (UV) light is a type of

ionizing radiation which can damage

DNA. When UV light strikes DNA,

it can cut one or two strands, leaving

two types of breaks in the DNA.

This breakage is particularly ap-

parent when using plasmid DNA,

since it is in the form of a circle. If

one strand of the plasmid DNA is

struck by UV light, an open circle

results. If both strands of the DNA

are cut, a linear form of DNA results. The uncut DNA remains in the supercoiled form, as

illustrated in Figure 1 above. When run on a gel, the results look like Figure 2 at right.

Experiment

Name: Class: Date:

Figure 1 Illustrations of uncut and cut DNA.

Open Circle Linear

UncutSupercoiled

DNA

OriginDirection of Movement

Voltage Variation

Figure 2 The uncut DNA moves most quicklythrough the DNA, followed by the linear circle andlinear species.

68 The Effect of UV Light on DNA Laboratory No. 15

68

Review Questions1. What kind of radiation is UV light?2. Why is plasmid DNA used to study the

effects of UV light on DNA?3. If DNA remains uncut, what form results?4. If one strand of DNA is cut, which form

results?5. If two strands of DNA are cut, which form

results?6. Draw a gel and place the 3 forms in order

on the gel.

Procedure - Day 11. Cast 1% agarose gels and place them in

the gel boxes.2. Pour TBE buffer until the level just

immerses the gels.3. Remove 5µl from the UV tube , add 1 µl

of loading dye, and place the sample inwell 1 on the gel for UV light. Repeat thisprocedure for the control group.

4. Place the open 1.5 ml tube 8 cm belowthe UV light and position it. Put on goggles and then turn the lamp on.

5. After 10 minutes, remove another 5 µl sample, add 1 µl of loading dye, and place it inwell 2 on the gel. Repeat this procedure for the control group.

6. Continue taking 5 µl samples, adding loading dye, at 10 minute intervals up to50 minutes for both the UV experimental sample and the control sample.

7. After the last sample has been taken, seal the gel boxes, and turn the power supply to22 volts.

8. Let the gels run for 12–13 hours.

Procedure – Day 29. Stain the gels with either ethidium bromide or methylene blue, or other DNA stain.10. Take a picture of the gel.11. Enlarge the picture on a copy machine 200%.12. Measure the square area of each band and record it.

Analysis of Experiment1. Find the square area of linear, open circle, and supercoiled (uncut) DNA in each band.2. What happens to the percentage of supercoiled DNA as the time of UV irradiation

increases?3. What happens to the percentage of linear and open circle DNA as the time of UV

irradiation increases? Why?

UV Light

Ice

12 cm.

Figure 3


69


Which Wavelength Causes PhotograyLenses to Change Color?

GoalTo determine which wavelength (color) of light causes photogray lenses to change color.


• Explain which wavelength causes Photogray glasses to turn dark.

Background InformationPhotogray lenses absorb UV light which causes them to change to a darker color, providingan automatic sunglass effect.

Helpful Hints• This activity can be done as either a demonstration in front of class or as one lab station

among several.• Make sure the lenses have been kept in the dark before the lab/demonstration to ensure

that they are clear.• The darkening reaction in the lenses should begin quickly after exposure to UV light.

ExtensionsUsing a Photodetector measure the light absorbed by the photogray lenses when clear andwhen dark. Determine absorbtion characteristics of the lenses.

Experiment


70


71

Laboratrory No. 16:

Which Wavelength Causes PhotograyLenses to Change Color?

PurposeTo determine which wavelength (color) of light causes photogray lenses to change color.


2 photogray lenses; UVA, UVB lamps; IR lamp; red, orange, yellow, green, blue, violetfilters; lamps for the filters; clay or other stand for the lenses, opaque box with side cut out tohold lens.


Discussion

Photogray lenses contain a substance which causes them to turn darker when exposed to

sunlight, and clear again when the sunlight is gone. Sunlight consists of many different

wavelengths of light (color), one or several of which could cause the photogray lenses to turn

dark.

Hypothesis

1. Which wavelength of of light do yoususpect will have the greatest effect on thephotogray lenses? Why?

Procedure

1. Set up the photogray lenses on its standand place it in a box with the open sidepointed in (see Figure 1).

2. Place the filtered lamp in between thelenses pointed away from each other.

3. Position the lamp 25 cm from the lens andturn it on.

4. Wait 30 seconds to see if there is a reaction.

5. Continue using all of the lamps until a reaction is seen.


1. Which wavelength of light turned the lenses gray?

2. What kind of chemical reaction is occuring in the glass?

Experiment

Name: Class: Date:

Photogray Lens

Filter

Filtered Light SourceUV,IR Lamps

25cm.

Figure 1

72 Photogray Lenses Laboratory No. 16

72


73


Which Wavelength CausesSunrez® to Solidify?

GoalTo introduce students to a light-dependent chemical reaction.


• Determine which frequency in the electromagnetic spectrum causes Sunrez to solidify.• Graph the results accurately.• Explain why goggles must be worn around UV light.• Distinguish between the wavelengths associated with UV, IR, and visible light.

Background InformationSunrez® is an unsaturated polysester resin with a boiling point of 294°F and a flashpoint of70-80°F. It is a photocuring resin which will only soldify with exposure to a photo initiatorsuch as UVA. It reacts just beyond the visible range at 380–410 nanometers. This resin mayabsorb other frequencies but primarily cures best in the UVA range.


UV light can damage the retina.• The resin must be poured and kept in a well-ventilated area during the entire experiment.

Helpful Hints• Sunrez® or a similar photo-curing adhesive resin can be obtained at auto windshield

repair shops.

Analysis of ResultsStudents should find that only in UVA and somewhat in UVB will the resin harden. It willbecome viscous in the other frequencies, and even in the dark, but only in UVA/B will itsolidify. If direct sunlight is used as a positive control, it will of course harden. This shouldbecome apparent to the students as the force to pull the popsicle stick increases in UVA/Band remains low and constant in the other wavelengths.

Experiment


74


75

Laboratory No. 17:

Which Wavelength CausesSunrez® to Solidify?

PurposeThe purpose of this lab is to determine which wavelength of light causes Sunrez to turn froma liquid to a solid.


Sunrez® or similar photo-curing adhesive resin, UVA lamp, UVB lamp, infrared lamp; red,orange, yellow, green,blue, violet filters; dark area, lamps for each color, goggles, 10 smallplastic drinking cups, spring scales, popsicle stick. Direct sunlight can be used as a positivecontrol if it is sunny on the day of the experiment. Simply add one more group and one moresmall plastic drinking cup/ popsicle.

Caution: Always wear plastic goggles when working with UV light. Sunrez is caustic; avoidcontact with skin and eyes.

Discussion

Certain types of adhesives and resins such as Sunrez® turn from liquid to solid in the presenceof certain wavelengths of light. This process is called photocuring and is typically used tojoin optical components. As such, it is considered a photocuring resin. Sunrez® is used torepair car windshields.

The sun emits energy at many different wavelengths, including infrared, visible, andultraviolet.

Review Questions

1. What kind of resin is Sunrez®?

Procedure

1. The class will be divided into 10groups, one for each wavelengthof light and one control group(dark).

2. Coat the inside of the cup withvaseline.

3. Pour 2 cm of resin into yourgroup's cup and place a popsiclestick into the middle.

4. Attach a spring scale sidewaysto the popsicle stick and pull.Record the grams of resistancefor your color.

Experiment

Name: Class: Date:

Light Source - UV, IR

Filter

Spring Scale

Resin

Figure 1

76 Which Wavelength Causes Sunrez to Solidify? Laboratory No. 17

76

Procedure (continued)

5. Place your sample under your lamp so the surface of the resin is 15 cm away from thelamp. Turn the lamp on.

6. Every 2 minutes reattach the spring scale and pull to determine the grams of resistance.

7. Continue taking readings every 2 minutes for 30 minutes.

8. Record your data on the board, and copy the entire data table.

9. Graph the change in viscosity (grams of resistance) for each wavelength (color) vs. timeon a graph.

Analysis

1. Which wavelength(s) worked best to solidify Sunrez?

2. How do you know?

The Electromagnetic SpectrumRadio Waves

77

Instructor’s Guide to Lab Nos. 18-23:

Radio and Microwave ExperimentsGoalTo familiarize students with radio-frequency electromagnetic waves.

ObjectivesAfter doing these radio/microwave activities the student should:

• Understand the fact that light, radio, television, and other forms of electromagnetic wavesare really only one phenomenon.

• Be able to calculate the wavelength of electromagnetic waves given the frequency orcalculate the frequency given the wavelength.

• Better understand how a diffraction grating works by adding waves that are in phase andproducing strong signals in certain directions.

• Be able to estimate the wavelength of electromagnetic waves being used in a certainapplication just by looking at the sizes of the structures involved.

• Have an idea that generation of electromagnetic waves by acceleration of chargedparticles, usually electrons, and detection of these waves by their effect on free electronsas the wave passes are just reciprocal processes. Appreciate that an electric field, whetherproduced by a battery connected to a wire or a traveling electromagnetic wave, willaccelerate charges such as electrons.

Background InformationA full description of each Radio Science lab is presented in the Student Activity sheets.

Many of these lab activities are intended as home projects, such as Lab No. 20 whichrequires that the student tune in an AM radio signal at night in order to detect changes in theEarth’s ionosphere between day and night.

Helpful Hints• The Resource Box contains four crystal-radio kits for students to build for Lab No. 19.

The crystal radio consists of a simple resonant circuit that can be tuned to AM radio-frequencies to detect radio waves with an antenna consisting of a long wire.

• You may want to get a cheap amplified-speaker setup that can used to amplify a crystalradio for the entire class to hear. These can be obtained for about $10 at computer stores.

• Crystal radio kits can be obtained through science education suppliers such as Frey orCenco. A crystal-type radio can be built from common electronic components bywrapping your own tuning coils using copper wire around an ordinary 35 mm filmcanister. See Figure 1 below for details.

• An inexpensive AM/FM radio is included in the Resource Box. This radio can be used asthe signal detector in the radio-wave diffraction experiment (Lab No. 22). It is importantthat the radio detector does not have Automatic Gain Control (AGC). This is a specialcircuit that even many inexpensive radios have to compensate for varying receivingsignal strengths. Since the experiment requires that students detect differences in signalintensity (strength), the AGC circuit will offset this effect and ruin the experiment. Wehave found RadioShack #12-734 to work well for this.

Experiment

78 Instructor’s Guide to Lab Nos. 18-23

78

• Some wire-mesh screening is included in the RB to make a Farady-cage. Form this intobox-like cage and place the radio in it to demonstrate the shielding of electromagneticwaves.

• We have put together ONE Microwave Transmitter/Receiver apparatus so far. Thisapparatus will be loaned out to one Resource Box unit at a time until we can build more.

References

1. Forest M. Mimms, Engineers Mini-Notebook: Communications Projects, pg. 34-35,(1994.) Available at RadioShack stores.


79

Laboratory No. 18:

Measuring the Length of Radio WavesPurposeTo investigate the relative wavelengths of electromagnetic waves.

Required Equipment and SuppliesCar with an AM/FM radio, someone to drive it, and a tunnel.

DiscussionAll electromagnetic waves travel at the speed of light c which is equal to the product of

the length of the wave (wavelength) λ times its frequency f , written as

c f= ⋅λIf we measure frequency in cycles per second (hertz, or Hz), and wavelength in meters (m),then the speed of light will be measured in meters per second. Its measured value is veryclose to

c = ×3 108 m/s

which is scientific notation for three with eight zeros after it, or 300,000,000 m/sec. This isabout 186,000 miles per second! All electromagnetic waves travel at the same velocity invacuum, the speed of light. If the waves are traveling through some material like air or glass,they may travel at some other slower speed. In this case the frequency stays the same as itwas in vacuum, but the wavelength decreases.

The relationship c f= ⋅λ says something very interesting. It says that if the frequency ofthe wave f gets higher, the wavelength λ must get shorter, since λ = c f/ . This means thatthe waves used for the FM band are about a hundred times smaller in wavelength than thewaves used for the AM band. When you listen to KFMB AM 760, the 760 means that thefrequency of the electromagnetic waves being broadcast by the station is 760 kilohertz, or760 thousand cycles per second. For this frequency the wavelength is:

λ = = ××

=c

f

3 10760 10

3958

3

m/s cycles/s

m

When you listen to KPBS, the frequency is 89.5 MHz, or 89.5 million cycles per second. ForKPBS the wavelength is:

λ = = ××

=c

f

3 1089 5 10

3 358

6

m/s cycles/s

m.

.

Your microwave oven operates at a frequency of 2.45 GHz (gigahertz), which is 2.45thousand million cycles per second. For your microwave oven, then, the wavelength is:

λ = = ××

=c

f

3 102 45 10

0 1228

9

m/s cycles/s

m.

.

So we have familiar electromagnetic waves, which we use every day, with wavelengthsranging from the length of four football fields to about the size of a dollar bill. What aboutlight waves for comparison? The red light from a He-Ne laser has a frequency of 474 million-million cycles per second, so its wavelength is:

λ = = ××

=c

f

3 10

474 100 0000006328

8

12 m/s

cycles/s m.

Experiment

Name: Class: Date:

80 Measuring the Length of Radio Waves Laboratory No. 18

80

This wavelength is very small, so small that we typically measure light in nanometers (nm),which is 109 meters. But you can see light and you have probably done some experimentswith light like dispersing it with diffraction gratings. Now we'll try some experiments withradio waves that show that they behave just like light but with much bigger sizes ofexperimental objects to handle the much longer wavelengths

It should be clear to us by now that the main difference in the types of electromagneticwaves we have investigated, including visible light, microwaves, ultraviolet, and infrared, isthe wavelength (or frequency) of the waves. After all, radio waves can be thought of as justbeing long-wavelength light waves (or light can be thought of as being super-short radiowaves!). Because of their different wavelengths, radio waves interact with matter somewhatdifferently than light waves, although both certainly display the same type of electromagneticwave behavior – like interference, diffraction, and reflection – but on a different size scale.Similarly, because of their size difference, carrier waves used by the AM band interact withmatter slightly differently than FM carrier waves because of their great size difference.

Although electromagnetic waves travel freely through space, they can get balky when wetry to confine them. They can pass through a tube if the diameter of the tube is severalwavelengths or more, but they cannot pass if the diameter gets comparable to a wavelengthor smaller. This fact permits us to find out what the wavelengths of electromagnetic wavesare just by looking for them in confined areas. For example, since the wavelength of thewaves used in microwave ovens are about 12 centimeters long, the small holes in the door ofyour microwave prevent the passage of the high power microwaves you use to cook and yetpass the shorter wavelength visible light.

Review Questions

1. What is the speed of all electromagnetic waves traveling in a vacuum?2. How is this speed related to the frequency and wavelength of an electromagnetic wave?3. Which waves are longer, visible light or the waves used in a microwave oven?

Procedure

1. There are lots of tunnels around on our freeway system, one good one goes under I-5 nearthe airport, but you can get the effect even if you only go under the freeway at anoverpass. Drive into the tunnel with your radio at 760 AM and then do it again using thefm band. The AM signal will get weak or fade out altogether in the tunnel, but the fmsignal should stay strong with almost no fade. What is the difference between the twosignals in terms of their wavelengths? (See the calculation done in the discussionsection.) Can you see through the tunnel? (Note: All good car radios have a circuit calledAutomatic Gain Control, which tries to compensate for changes in received signalstrength by boosting the gain when the signal goes down. This circuit competes with theeffects this experiment is trying to demonstrate, so do not be discouraged if you have tolook for just the right tunnel. It will be better to use a relatively weak radio station, so 760works well in north county, but you might have better luck with a Los Angeles stationlike 1070 in areas closer to downtown San Diego. If necessary, get a cheap AM/FM radiowhich doesn't have this AGC circuit.) What is the wavelength of light compared to thetunnel diameter?

2. Measure the size of the holes in the door of your microwave oven and tell what you canconclude from your measurements about the wavelength of light and the wavelength ofthe microwaves you use to cook.


81

Laboratory No. 19:Tuning Into Radio Waves

PurposeTo understand the principles of radio communication.

Required Equipment and SuppliesCrystal radio kit.

DiscussionRadio and television sets operate with electromagnetic waves generated by com-

mercial and public stations. Here’s a brief explanation of how a radio wave is broadcast atone location and received at another.

If you stand at the edge of a pond of still water and shake the end of a stick back andforth in the water, you’ll produce waves on the water surface. These waves will spreadoutward as they travel away from you, weakening as they spread. If your friend is standingnot to far away at the other edge of the pond, she will see the waves you created at the otherside. She might watch a small piece of wood floating on the surface bob up in down in thewaves. Using these waves you can actually send messages to your friend by slightlymodifying, or modulating, the way you shake the stick. One way would be to shake harder orsofter, causing your waves to be bigger or smaller. Your friend will see correspondingincreases and decreases in the amplitude of the bobbing wood. This method of sending amessage is called amplitude-modulation, or AM. Another way to send information would beto slightly change the rate at which you shake the stick, which would slightly change thefrequency of your waves and the oscillation rate of the floating wood.

This is the principle behind radio and lightwave communication. Of course we don’t usewater waves to broadcast our favorite music and important information; we use electro-magnetic waves. It works in a similar way, though. If you shake an electrically-charged rodto and fro in empty space, you’ll produce electromagnetic waves in space. This is because themoving charge is actually an electric current. What surrounds an electric current? The answeris a magnetic field. What surrounds a changing electric current? The answer is a changingmagnetic field! In this way the vibrating electric and magnetic fields regenerate each other tomake an electromagnetic wave, which moves outward from the vibrating charge at the speedof light.

This is essentially how a radio transmitting antenna sends out a wave. Rather than shake alarge charged antenna, however, we shake the electrons inside the metal of the antenna usingoscillating electric currents. These electric currents are generated by a transmitter, and theoscillation rate determines the frequency of the electromagnetic waves that are sent out.Every radio station has an assigned frequency at which it broadcasts; the electromagneticwave transmitted at this frequency is called the carrier wave. This carrier wave is eitheramplitude or frequency modulated, depending on the type of radio station broadcasting. AMstations broadcast in the range of 535 to 1605 kilohertz (thousands of waves per second),while FM stations broadcast in the higher frequency range of 88 to 108 megahertz (millionsof waves per second). Both of these electromagnetic frequency ranges are like very low-frequency light waves. Amplitude modulation can be thought of as changing the brightnessof a constant color light bulb. Frequency modulation is like changing the color of a constant-intensity light bulb.

Experiment

Name: Class: Date:

82 Tunign into Radio Waves Laboratory No. 19

82

With both AM and FM the carrier wave is modulated by electrical currents we call thesignal. The signal is produced by converting sound vibrations from a voice or musicalinstrument into electrical vibrations (currents) using a microphone at the radio station.

As a radio wave leaves the transmitting antenna, it spreads out in all directions. Theoscillating electric field of the wave causes the movable electrons in a distant radio’s metalantenna to oscillate. These electrons dance a jig that is a miniature version of the electronmotion in the transmitting antenna. Although the signal weakens as the energy of the wave isspread out over a larger and larger distance, radios that aren’t too far away can pick up thesignal. This signal is very weak and is usually amplified and then sent through an electricalcircuit that reconstructs the original sound from the radio station and plays it through aspeaker or headphone. It turns out that just about any piece of metal can be used as areceiving antenna. In fact, each metal pot and pan in our kitchens is receiving radio signals,causing the electrons within them to oscillate with the incoming radio waves. Of course ourpots and pans don’t usually have the circuits, amplifiers, and speakers required to processthese electrical signals and replay the sound being broadcast, though.

Since radio waves are generally coming in from many nearby stations at the same time, aradio must be able to be adjusted, or tuned, to pick out only one carrier signal at a time fromthe many being received by the antenna. This is done by an electrical circuit called a tunerusing a process called resonance. A resonant circuit is easy to build using common electricalcomponents. We can build a simple AM radio using a capacitor, an inductor, and a one-waycrystal to detect and tune into some of the many radio waves surrounding us.

Review Questions1. Are sound waves part of the electromagnetic spectrum?2. What is meant by modulation?3. What is the difference between AM and FM?

Procedure1. Assemble the crystal radio included in the supply kit.2. Examine the device closely and try to understand how it operates. Look for an external

source of power, such as a battery or power chord.4. Attach the antenna leads to a large metal object.3. Connect your earphone to the radio and try to tune in a local AM station. If no signal is

obtained, try stringing several meters of electrical wire around the room as an antenna.4. Listen to the broadcast until the station is identified (this is done regularly throughout the

day). Place a piece of masking tape along the length of the coil and note the position ofthe tap on the tape. Write the stations call number at this point.

5. Reconnect the antenna leads to a larger piece of metal. Note whether there is a change inthe strength of the sound.

Analysis of Experiment1. The crystal radio has no battery! Where does the power come from?2. What role does the tap position play in the circuit?3. What is the frequency of the station you detected and tuned into? Remember, AM

frequencies are given in kHz.4. Electromagnetic waves travel at the speed of light c which is equal to the product of the

length of the wave λ times its frequency f , written as c f= ⋅λ . What is the length ofthe wave you tuned into?

5. What is the wavelength of the carrier wave used by your favorite FM station?Remember, FM frequencies are given in MHz, so KPBS at 89.5 uses 89.5 million cyclesper second wave to broadcast its signal.


83

Laboratory No. 20:

The Shielding of Radio Waves by MetalPurpose

To understand the principles of radio communication.

Required Equipment and SuppliesPortable AM radio, aluminum foil, small cardboard box.

Discussion

Radio waves (as well as many other electromagnetic waves) move electrons only at thesurface of metal; electric fields can’t penetrate metal because the mobile electrons at thesurface will quickly move around in such a way as to cancel out the electric field at thesurface. Instead, the oscillating electrons at the surface absorb a wave and then, because oftheir own accelerations, re-emit it in the action called reflection. Metals reflect radio wavesjust as a mirror reflects light. A radio won’t play inside a closed metal box. Lightning can’tpenetrate to the inside of a metal car.

Surprisingly, radio waves pass right through our bodies all day, every day. It’s easy toprove by putting a small portable radio on the ground, and laying over it, the radio still plays!Radio waves don’t greatly disturb the electrons in any non-conducting matter because thewaves carry very little energy and the electrons aren’t free to move around in anonconductor. Compared to metals, humans are very poor conductors of electricity. We don’tdisturb the incoming waves much. Although nonconductors can shield electromagneticwaves by absorbing them, the material must be at least several times thicker than thewavelength of the wave for total absorption. Since our bodies and many of thenonconduction objects around us are thin compared to the wavelength of radios waves, radiowaves can pass through a roof, a wall, or a person without much absorption.

For radio communication, radio-frequency interference can be a big problem. Just aboutevery electronic circuit that has changing or oscillating currents in it will radiateelectromagnetic radiation, a lot of it in the range of frequencies that includes radio and TV.Most electronics products have to be shielded so that they don’t interfere with other devicesor disrupt radio and TV communications. This is why so many electronic products carry alabel that shows that the device is approved for home use by the Federal CommunicationsCommission (FCC). Computers especially produce a lot of electromagnetic radiation becausethere are so many electrons being accelerated in the circuits as the computers do calculations.If you take a radio which is not tuned to any station and hold it near to a computer, you willbe amazed at how strong the signal is. You can actually hear it work, just like the CIA tries tohear the computers working in foreign embassies. You have to try to put a metal shield boxaround all the radiating circuits to keep the electromagnetic radiation inside the computer ifpossible and if you look inside the computer, you will see that the designers did just that.Nevertheless, your computer is a rather good radio transmitter. Embassies often put theirsensitive computers in rooms with metal walls so the eavesdroppers are stymied. In this caseit is necessary to use old fashioned spies like James Bond and Matta Hari!

Activity

Name: Class: Date:

84 The Shielding of Radio Waves by Metal Laboratory No. 20

84

Review Questions

1. Can radio waves pass through metal?

2. Can radio waves pass through people?

3. Explain why your answers to questions one and two may be different.

Activities

1. Find a small cardboard box that the portable radio will fit inside. While the radio isplaying and tuned into a strong station, put the radio inside the box and close the box.Can you still hear the radio playing? Of course the sound coming from the radio ismuffled, but is it still receiving the station? Now completely cover the box withaluminum foil, or place the radio in a cage made from wire mesh. Is the radio stillplaying? Poke some large holes in the box. Is it still playing now? What does this tell youabout the shielding of radio waves by cardboard versus the foil? Why are they different?The free electrons in the metal oscillate with the incoming wave and cancel its electricfield inside the metal. If you ship an audio tape across country to a friend, you mightwrap it in aluminum foil to keep any stray electric fields from damaging the tape intransit.

2. Use a portable AM radio to detect the radio-frequency emissions of a computer. Tryusing an AM radio tuned to about 550 kHz, since there is usually no radio stationtransmitting at this frequency. Have the computer do something like read a file intomemory. Notice what you hear. Take the radio to other nearby electronic devices andcircuits that you suspect might be emitting radio waves. Make a note those that generateradio-frequency “noise” (at 550 kilohertz or so).

3. When electromagnetic waves penetrate into some material which absorbs them strongly,they can be detected about one wavelength into the material almost no matter how strongthe absorption. Sea water strongly absorbs electromagnetic waves. If you wanted tocommunicate with a submarine which was submerged, would use short or long radiowaves? It turns out that the wavelength of electromagnetic waves in water is shorter thanit is in air, so you will need even lower frequencies than you might think for this job. Infact, you will need frequencies of only a few hertz, or a few cycles per second. Becauseyou need quite a few wavelengths of the wave to pass before you can tell that you arereceiving an electromagnetic wave and interpret either the AM or FM signal which issuperimposed on the wave, this means that the amount of information you can transmit isquite limited ...t hings like “launch,” “don’t launch,” or “good-bye” are about it.


85

Laboratory No. 21:

Using the Earth’s Ionosphere toReflect Radio Waves

PurposeTo investigate the long distance transmission of AM and short-wave radio waves viareflection waves by the earth’s ionosphere.

Required Equipment and SuppliesPortable AM radio or AM car radio.

Discussion

In the thin gas of the earth’s

atmosphere, above the ozone layer,

strong ultraviolet radiation from the

sun ionizes (breaks apart into charged

particles) some atoms and molecules,

creating what is called the ionosphere

(Figure 1). These ions (charged

particles) respond to the oscillating

fields of AM radio waves rising from

the earth’s surface, causing them to be

reflected back to earth. This is allows

short-wave radio transmissions and

some AM radio stations to be

received over great distances without

a straight line of sight to the

transmitters. The ionosphere does not reflect FM radio and TV signals, however. Their

frequencies are so high that the ionosphere’s electrons can’t respond fast enough to the

changes in the electric field of those waves to reflect them back to earth.

Review Questions

1. What produces the Earth’s ionosphere?

2. Why are AM radio waves reflected off the ionosphere while FM are not?

3. Why can short-wave radio operators communicate with one another half way across the

world?

Activity

Name: Class: Date:

AM

FM

Ionosphere + – + –+ –

+ –+ –

+ –

+ –

Figure 1

86 Using the Earth’s Ionosphere to Reflect Radio Waves Laboratory No. 21

86

Activities

1. Some night when you are in a car, tune the AM radio to the farthest station you can find

on the AM band. Listen for the call sign of the station and what city it is broadcasting

from (note that you will never find an FM signal transmitted from so far away, since they

are not reflected off the ionosphere like the longer AM waves). Then leave the dial

untouched and return to the car the next morning. You won’t be able to receive the

station. What you are indirectly demonstrating is the daytime/nighttime change in height

and ion concentration of the ionosphere. The solar UV radiation that forms the ions in the

daytime is absent at night, allowing many of the ions to begin to recombine into neutral

atoms. This recombination into atoms happens fastest where the ions are closer together,

at the lower (and hence more dense) edge of the ionosphere. As night falls, it is as if the

lower boundary of the ionosphere rises, and the reflected AM radio waves can go farther

since they are reflected at greater heights, as shown in Figure 2.

Nightime IonosphereBoundary

Daytime IonosphereBoundary

+ –+

–

+ –

+ –

AM

Figure 2


87

Laboratory No. 22:

A Diffraction Grating for Radio WavesPurposeTo understand the principles of wave diffraction using radio waves.

Required Equipment and SuppliesFive or four cars. Portable FM radio (cheap, without automatic gain control (AGC) circuitry);Permission for field trip!

Discussion

In earlier lab activities you used a diffraction grating to disperse visible light by creating

an interference pattern. Recall that the interference pattern was formed because of the effects

of diffraction and interference of light as it passed through a series of closely spaced slits.

The slits were spaced about one wavelength of the light apart. Both diffraction and

interference effects are general properties exhibited by all electromagnetic waves, including

radio waves. In theory, then, we should be able to make a diffraction grating that works for

radio waves! Since the wavelength of radio waves is much longer than light, though, the

dimensions of the radio grating have to be HUGE. So how exactly can we make a diffraction

grating that works for radio waves? Well, we need something that screens out the radio

waves (metal) and which can have gaps in it about a wavelength apart. The wavelength of

FM radio waves is about a meter. So what can we use as a diffraction grating? Cars!! Check

out the diagram below to see how it might be done.

Los Angeles

Incoming Waves

1 meter

Cars

FM Radio Strong Signal

Weak Signal

Strong Signal

Strong Signal

Weak Signal

Review Questions1. What type of device is used to form an interference pattern?

2. What two wave properties cause electromagnetic waves to form an interference pattern?

2. Which electromagnetic wave has a longer wavelength – radio waves or visible light?

3. Why is it so difficult to make a diffraction grating for radio waves?

Activity

Name: Class: Date:

88 A Diffraction Grating for Microwaves Laboratory No. 22

88

Procedure1. For this activity we have to choose the location carefully, since radio waves bounce off

metal objects just the way light bounces off a silvered mirror. A path over water is goodto avoid this, so try parking three cars at the Moonlight Beach parking lot.

2. Park the cars nose to tail with about 1 meter gap between them. Try to orient the cars sothe direction to Los Angeles is perpendicular to their line.

3. Tune the small portable radio to a Los Angeles FM station.

4. Move away from the cars so they are between you and Los Angeles. Go about 6 metersaway, as shown in the diagram above.

5. Now walk parallel to the line of cars and note what happens to the signal strength. Thereshould be positions of strong signal and positions of weak signal. You have constructed adiffraction grating for radio waves which works just like the one for light, but in the caseof radio, the dimensions have to be HUGE.


89

Laboratory No. 23

A Diffraction Grating for MicrowavesPurposeTo understand the principles of wave diffraction using microwaves.


Microwave transmitter and receiver, oscilloscope or voltmeter, aluminum foil, cardboardscreen, tape.

DiscussionWe have examined the phenomena of diffraction and interference in great detail in

previous labs. Why do it with microwaves? Well, first of all we may be interested in seeinghow microwaves are and are not like electromagnetic waves in other regions of the spectrum.Second of all, the wavelength of microwaves turns out to be an excellent size for interferenceand diffraction experiments. Recall that the spacing in a diffraction grating must be on theorder of the same length of the waves to be diffracted. For light, this is tiny, so small that thewavelengths are measured in nanometers (10–9 m). Great care must be taken in order to makea diffraction grating with any kind of precision. On the other hand, we have seen that radiowaves can be several meters long or so. This size is also hard to make a diffraction gratingfor. Microwaves, however, have wavelengths measured in centimeters, which make themeasy to experiment with. In fact, we can make a precision diffraction grating out of aluminumfoil and cardboard using scissors!

It is also easy to make totally and even partially reflecting “mirrors” out of wire mesh.This makes microwaves an especially well-suited for demonstrating interference using whatis known as the Michelson-Morely method. This name is given to a special interferencearrangement that helped lead to the development of the Theory of Relativity! Your instructorcan give you the details – let’s get down to our microwave experiment!

Cardboard

Aluminum Foil2 cm.

10 cm.

MicrowaveSource

Scope orVoltmeter

Detector

It would be fun to do this experiment using the high power microwaves in yourmicrowave oven. You could cook stripes into a piece of pizza dough, for example. But,unfortunately, no mother wants her kid to come home with stripes, so we have to do thisexperiment with a small microwave source. These are rather expensive, but some sources anddetectors which are low power and are safe to use have been made available. Thesetransmitters and receivers are surplus units from microwave systems and use a Gunn diode

Experiment

Name: Class: Date:

90 A Diffraction Grating for Microwaves Laboratory No. 23

90

both as the transmitter and as the local oscillator for the receiver. The power is only about30 mW.

Review Questions1. Why are microwaves useful for interference and diffraction experiments?

2. Which is longest and which is shortest, microwaves, radio waves, or visible light?

3. About how long is a microwave?

foilgap

θd

λ

IncomingWaves

Detector

Procedure1. Make a transmission diffraction grating using a piece of cardboard and aluminum foil.

Space strips of foil so that there is about a 2 cm gap between them. and put about 5 or 10strips on the cardboard. Each strip should be about a wavelength wide (about 10 cm at2.5 GHz) and all strips and all gaps should be the same width.

2. Illuminate the grating with the microwave source and move the detector back and forthon the other side of the grating looking for maxima and minima in signal strength. Thereshould be periodic maxima and minima. If you can still detect the signal, go far awayfrom the grating and measure the angle to the first maximum off from perpendicular. Thiscan be done using trigonometry. Measure the perpendicular distance of the detector fromthe centerline (x) and the length along the centerline (s). It can be seen that

sinθ θ= =+

side opposite hypotenuse

x

s x2 2

3. This angle is the one where the waves from one slit have to go just one wavelengthfarther on their way to the detector than the waves from the neighboring slit, so they allarrive at the detector “in phase” or such that they all add up, creating a strong microwavesignal.


Calculate the wavelength at which your microwave source is operating from the fol-lowing equation, which works for optical diffraction gratings and for your microwaveone made from foil:

λ θ= d sin

where d is the width of your foil strips (actually the distance between the centers of theareas between the foil strips) and θ is the angle off-perpendicular to where the detectorsees its first maximum.

The Electromagnetic SpectrumResource Box

91

The Curriculum Resource Box

Below is a list of the items included in the Resource Box developed for use with The Electromagnetic Spectrumcurriculum. Each item is packaged with a label for identification and repacking. Operating instructions andspecifications of the individual equipment is reproduced in the section following. When you are finished withthe Resource Box (RB) and are ready to pass it on, please repack it carefully. Feel free to contact Dr. DanielFinkenthal at (619) 455-4664 or [email protected] for assistance. Good luck and enjoy!

List of Contents:ResourceBox # Item Description Q Source

RB-01 UV Lamp (Mineralight UVG-11) 1 Frey #F14062

RB-02 Mercury Vapor Light Source 1 Frey #F23384

RB-03 Intense Light Source 1 Frey #F01910

RB-04 Video Monitor 1 Gateway

RB-05 Video Camera 1 Gateway

RB-06 Clamp Type Lamp 1 Frey #F00223

RB-07 Economy Hot Plate 1 Frey #F01953

RB-08 Variable AC Dimmer (Triac) 1 Finkenthal

RB-09 UV Goggles 4 Frey #F14583

RB-10 Electroscope 1 Frey #F02678

RB-11 Fluorescent Mineral Collection Frey #F08934

RB-12 Flame Test Salts (set) 1 Frey #F02685

RB-13 Optical Slits Kit 1 Frey #F01920

RB-14 Scale and Slit 1 Cenco 86260-01

" Metal Suports Set (for ob) 1 Cenco 85850-01

" Screen Support 1 Cenco 85970-01

" Mercury Thermometer(10 to 110 x 1°C)

1 Frey #F12885

" X-Acto Knife 1 GA

RB-15a Crystal Radio Kits (Small Parts) 4 Frey #F991138

RB-15b Crystal Radio Kits (Bases) 4 ''

RB-16 Curriculum Manual 1 GA

" Holographic Diffraction Grating (large) 2 Learning Technologies

" Latex Gloves 5 pr GA

" Color Filters (Red, Blue, Green) 2 Edmund C35,135

RB-17 Portable AM/FM Radio 1 Radio Shack 12-734

RB-18 Wire Mesh for Faraday Cage 1d Finkenthal

RB-19 Compact Disks (gratings) 30 GA

RB-20 Wheeled Tote Locker 1 HomeBase

Resource Box

List of Contents: The Curriculum Resource Box

92

RB-21 Radiometer 1 Frey #F990224

" Holographic Diffraction Dratings 30 Arbor Scientific

" Glass Prism 1 Frey #F990924

" Polarizers -2.5 in diameter 8 Edmund C38,396

" Calcite Crystal 1 Edmund C39,946

" Black Light Incandescent 1 Frey #F05285

" Clear/ Yellow/ Halogen/ Showcase Bulbs 1 ea GTE Sylvania

" Flourescent Crayons Set 5 Frey #F00231

" Invisible (fluoresent) Ink, Fluor. Markers 1 Frey #F02101

" Cobalt Blue Glass (4” sq.) 2 Frey #F07137

" UV Detector-Viewer/ UV Bandpass Filter 1 Finkenthal/ Oriel

" UV Detector Card 4 Science Kit

" IR LED Light Source 1 Finkenthal

" Lantern Mantel (Thorium Radiation Srce) 1 Sportsmart

" Microscope Slides 40 Frey #F14680

" Colloidal Graphite 1 Frey #F02641

" Cyalume Sticks -Assorted Colors 5 Edmund C37,218

Sources

Frey Scientific905 Hickory Lane, PO Box 8101Mansfield, OH 44901-8101Toll Free: 800-225-FREY(419) 589-1900 FAX: (419) 589-1522

Gateway Electronics, Inc of California9222 Chesapeake DriveSan Diego, CA 92123(619) 279-6802 FAX: (619) 279-7294

CENCO (Central Scientific Company)3300 CENCO ParkwayFranklin Park, Illinois 60131-1364800-262-3626 FAX: (708) 451-0231

ORIEL Corp.250 Long Beach Blvd., PO Box 872Stratford, CT 06497-0872(203)377-7877

Edmund Scientific Company101 E. Gloucester PikeBarrington, NJ 08007-1380(609) 573-6295 FAX: (609) 573-6295

Science KitPO Box 5059San Luis Obispoo, CA 93403(800) 828-9572

Sargent-WelchP.O. Box 5229, Buffalo Grove, IL 60089-5229800 727 4368 FAX 800 676 2540E-Mail [email protected]

Arbor ScientificPO Box 2750Ann Arbor, MI 48106-2750(800) 367-6695

Learning Technologies Inc59 Walden St.Cambridge, MA 02140(617) 547-7724

RadioShack (local outlets available)

HomeBase (local building supply retail outlet)

emt project 2

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